Tag Archives: #infectious disease

5 random facts about antimicrobials

Who doesn’t love to pick up random bits of information while they’re in line for their coffee or their morning signout? Here are 5 helpful pieces of information on antimicrobials to start off your day!

1.Cefepime vs. Piperacillin-tazobactam
Cefepime – cephalosporin
– DOES NOT cover gut anaerobes
– DOES NOT cover Enterococcus spp.
Piperacillin-tazobactam – penicillin derivative
– DOES cover gut anaerobes
– DOES cover penicillin-sensitive Enterococcus spp.

Antibiotic Cefepime Piperacillin/tazobactam
Class Cephalosporin Penicillin derivative
Gut anaerobic coverage? No Yes
Enterococcus coverage? No Yes (if susceptible)

 

2. Cephalosporins in general DO NOT cover Enterococcus spp.

3. Ertapenem vs. meropenem vs. imipenem vs. doripenem
Ertapenem – DOES NOT cover Pseudomonas spp.
Meropenem/Imipenem/Doripenem – DO cover Pseudomonas spp.
*None of the carbapenems cover MRSA

4. Ineffective antimicrobials
Daptomycin – inactivated by the surfactant in the lungs
– DO NOT use daptomycin to treat lung infections
*Remember: Linezolid, Lung (you can use Linezolid for lung infections)

Echinocandins (ex. micafungin, caspofungin, anidulafungin) – do not reach therapeutic levels in the urinary tract
– DO NOT use echinocandins to treat pyelonephritis or urinary tract infections

Tigecycline – accumulates in the tissues and has low concentration levels in the bloodstream
– DO NOT use tigecycline to treat bloodstream infections

5. Bone marrow toxicity due to linezolid increases after 2 weeks of exposure
– Avoid using linezolid for more than two weeks at a time when possible

 

Do you have any random facts of ID knowledge? Let me know in the comments section below!

 

REFERENCES:
1. Mandell, Douglas, and Bennett. Principles and practice of infectious diseases. Philadelphia, PA: Churchill Livingstone/Elsevier, c2010. 7th edition.
2. Zhanel, G.G. et al. 2007. Comparative review of the carbapenems. Drugs. 67(7):1027-1052.
3. Gerson, S.L. et al. 2002. Hematologic effects of linezolid: summary of clinical experience. Antimicrobial Agents and Chemotherapy. 46(8): 2723-2726.
4. Malani, A.N. et al. 2014. Candida urinary tract infections: treatment options. 5(2): 277-284.
5. Jeu, L. et al. 2004. Daptomycin: a cyclic lipopeptide antimicrobial agent. Clinical Therapeutics. 26(11): 1728-1757.

Peer-reviewed by Jeff Pearson, PGY-2 pharmacy resident

Oral vs. IV antimicrobials

What’s the difference between oral (PO) and IV medications? When do you use PO vs. IV antimicrobials? When are they interchangeable? These are the questions we’ll address in this post.

info intravenously picture
by Dalya Ferguson, MD

Bioavailability is an important concept to understand when considering IV to PO interchange.

Bioavailability = the measure of the amount of an orally administered medication that reaches the bloodstream.

Antimicrobials with >90% bioavailability are the antimicrobials we can target for
IV to PO interchange.

Antimicrobials where bioavailability >90%:
(therefore, can be switched to PO)

  • Metronidazole
  • Fluoroquinolones (levofloxacin, ciprofloxacin, moxifloxacin)
  • Trimethoprim-Sulfamethoxazole
  • Tetracyclines (doxycycline, minocycline)
  • Linezolid
  • Rifampin
  • Fluconazole/Voriconazole
  • Clindamycin
  • Azithromycin (only ~40% bioavailable, but the concentration achieved by
    oral ingestion is just as effective as IV for treatment)

 

IV medication = medication given intravenously
– medication takes effect immediately after the infusion
– administers a bolus of the medication quickly (within 5 minutes)
– requires an IV line
– bypass first pass metabolism in the liver

PO medication = medication administered per oral route
– medication takes effect in ~30 minutes to 6 hours
– requires ability to swallow, absorb the medication, and also undergoes
first pass metabolism prior to reaching the circulatory system

Why is PO preferable to IV?

  • Cheaper3
  • Does not require IV access
  • Easier and faster to administer
  • No IV complications (i.e. phlebitis, thrombosis, bloodstream infection)
  • Avoidance of a long-term catheter such as a PICC line
  • Less unnecessary fluid administration

 

When to consider IV antimicrobials?

  • when patient is unable to take PO or unable to absorb the medication
  • when you want immediate effect of the medication
  • when the spectrum of activity desired is only available with IV antibiotics
  • when no PO option is available to treat the pathogen
  • when PO medications will not achieve high enough concentrations or penetrations to the location of the infection
    • Critically ill patients; sepsis/bacteremia
    • Endocarditis
    • CNS/ocular infection
    • Osteomyelitis/Septic arthritis (*a study is currently under way, looking at whether certain oral antibiotics are non-inferior to IV antibiotics in
      bone infections7)
      *You may occasionally see these syndromes treated with oral antibiotics, because each case is different. But in general, consider these syndromes as ones where IV antibiotics are preferred, especially as initial therapy.

 

TAKE HOME POINTS:

  • IV antimicrobials are NOT “stronger” or “better” than oral antimicrobials
    – it depends on each individual medication
  • PO antibiotics should be used unless there is a reason to use IV antibiotics (and not the other way around)
  • When PO and IV versions of an antimicrobial are similar, make every concerted effort to make sure your patients are not on IV medications unnecessarily

 

Questions? Comments? Suggestions for future posts? Leave a comment below.

 

 

References:

  1. Kwong, L.H et al. (2015). An unsupported preference for intravenous antibiotics. PLoS medicine, 12(5): e1001825. DOI: 10.1371/journal.pmed.1001825
  2. MacGregor, R.R. et al. (1997). Oral administration of antibiotics: a rational alternative to the parenteral route. 24: 457-467. PMID: 9114201
  3. Chan, R. et al. (1995). Oral versus intravenous antibiotics for community acquired lower respiratory tract infection in a general hospital: open, randomized, controlled trial. BMJ. 310: 1360-1362. PMID: 7787537
  4. Baddour et al. (2016). Infective Endocarditis in Adults: Diagnosis, Antimicrobial Therapy, and Management of Complications. Circulation. 132: 1435-1486.
    DOI: 10.1161/CIR.0000000000000296.
  5. Tunkel, A.R. et al. (2004). Practice Guidelines for the Management of Bacterial Meningitis. CID. 39: 1267-1284. DOI: 10.1086/425368
  6. World Health Organization, Occupational Health. (date published unknown). Comparison of pharmacokinetics and efficacy of oral and injectable medicine [Powerpoint slides]. Retrieved from http://www.who.int/occupational_health/activities/5injvsora.pdf
  7. Li, H.K et al. (2015). Oral versus intravenous antibiotic treatment for bone and joint infections (OVIVA): study protocol for a randomized controlled trial. BMC Trials. 16:583. DOI: https://doi.org/10.1186/s13063-015-1098-y
  8. Wisplinghoff, H. et al. (2004). Nosocomial bloodstream infections in US hospitals: analysis of 24,179 cases from a prospective nationwide surveillance study. Clin Infect Dis. 39(3): 309-317. DOI: 0.1086/421946

 

Peer-reviewed by Jeff Pearson, 2nd year PharmD resident

Antimicrobials: spectrum of activity

One of the most difficult concepts to understand is the spectrum of activity of different antimicrobials. We are all taught each antimicrobial in silos of the other ones and I always found it difficult to create conceptual charts in my head. Thankfully, I’ve found some amazing charts on the internet (from reputable sources, of course) that shows antimicrobials in relation to one another that may be helpful to you.

Here, I will present the best ones I’ve found so far for antibiotics, antivirals, and antifungals for you to use as a reference guide. At the bottom, I will list a few caveats to take into account when using these charts, because as always, ID is never as simple as the charts imply.

Antibiotics

Antibiotic spectrum of activityIntensive Care Drug Manual: Wellington ICU. Appendix 5.

Re-writing the fine print of the chart in case it’s not easily readable:
*For simplicity, atypical organisms are not included above. Partial columns indicate incomplete coverage. ESBL-producing organisms are not susceptible to most antibiotics containing a beta-lactam ring; carbapenems are the usual agent of choice.
1: C. difficile should only be treated with metronidazole or vancomycin
2: ESCHAPPM are β-lactamase producing organisms. These are Enterobacter, Serratia, Citrobacter freundii, Hafnia, Acinetobacter/Aeromonas, Proteus (not mirabilis), Providencia & Morganella morganii. See my 1st post on SPICE organisms for more info.
3: Not effective against Clostridium

4: Metronidazole is not effective against Peptostreptococcus
5: Teicoplanin is not effective against Enterococcus faecium
6: Gentamicin is not appropriate mono therapy for Staphylococcus aureus & should only be used in conjunction with a β-lactam
7: Due to increasing MIC, Cefuxorime is not recommended therapy for Moraxella
8: Although it has other actions, Ceftazidime should only be used for Pseudomonas

*This chart is intended as a guide, pending specific identification & sensitivities – it does not replace expert ID advice. Local antibiotic sensitivities & preferences will vary.

My notes:
ClindamycinCommunity-acquired MRSA strains have been found to be resistant to clindamycin and thus, this is often not a safe option for empiric therapy against MRSA.
Rifampin
usually used as an adjunct with another antibiotic against most infection. Would not recommend its use in isolation against infections.
Co-trimazole/Trimethoprim
would not use against enterococcus or empirically against MRSA in the hospital/ICU
Moxifloxacin — has some anaerobic coverage while levofloxacin and ciprofloxacin do not.
Metronidazole – no longer the 1st choice for C.diff infection. Instead, use PO vancomycin or PO fidaxomicin. (Thanks to a commenter for pointing that out to me!)

 

Antivirals

This chart was made by me but inspired by Aliyah Baluch, MD, Msc from USF who did an amazing review of antimicrobials used in stem cell transplant recipients. I thought it was a great way to demonstrate the spectrum of activity of antivirals and I hadn’t seen anything similar prior to that. Check out IDpodcasts.net for other lectures on ID topics.

 

antiviral spectrum of activity

*This chart only covers the herpes virus family, and does not include other virus families
*Just because foscarnet and cidofovir are considered the most broad-spectrum of the bunch does not mean they are always the best options. These drugs are quite toxic and should only be used in special circumstances, often with the involvement of an ID specialist.

Antifungals

This is a great chart taken from a wonderful review on antifungals from Mayo Clinic Proceedings.

Screenshot-2018-3-18 Current Concepts in Antifungal Pharmacology - pdfLewis, R.E. Mayo Clin Proc 2011

References:

1.Lewis, R.E. (2011). Current Concepts in Antifungal Pharmacology. Mayo Clinic Proceedings. 86(8):805-817. DOI: 10.4065/mcp.2011.0247
2.IDpodcasts.org: Bugs, Drugs, and Stem Cells podcast. July 2017. http://idpodcasts.net/podcasts/bugs-drugs-and-stem-cells/
3.Intensive Care Drug Manual: Wellington ICU. Appendix 5. Updated 2017. http://drug.wellingtonicu.com/
4.Santos, C.A.Q. (2016). Cytomegalovirus and other beta-herpesviruses. Seminars in Nephrology. 36(5): 351-361.
DOI: 10.1016/j.semnephrol.2016.05.012
5.Razonable, R.R. (2011).
Antiviral Drugs for Viruses Other Than Human Immunodeficiency Virus. Mayo Clinic Proceedings. 86(10): 1009–1026. DOI:  10.4065/mcp.2011.0309

Peer-reviewed by Jeffrey Pearson, 2nd year pharmacy resident

 

 

CNS penetration of antimicrobials

Have you ever noticed how the indicated dosages for antimicrobials increase for CNS infections? This is because antimicrobials have a difficult time penetrating the blood brain barrier and the blood-CSF barrier, leading to difficulty of some antimicrobials to achieve therapeutic concentration levels in the CSF to properly treat a CNS infection.

Overview-of-the-two-main-barriers-in-the-CNS-blood-brain-barrier-and-blood

(Bhaskar et al. 2010.)

Disclaimer: the penetration of antimicrobials into the CSF is much more complicated than three columns and a list of antibiotics. It’s been shown that levels of drugs differ between ventricular, cisternal, and lumbar CSF. Additionally, the treatment of CNS infections depends on more than just the CNS penetration of a certain antimicrobial, thus if any questions arise, please discuss with your ID consultants and ID pharmacists.

For the sake of this review, we will keep it simple.

Antimicrobials can be broken down into 3 rough
categories:

Excellent/Good penetration of the CSF Good penetration only in inflamed meninges Poor penetration of the CSF
Fluoroquinolones Glycopeptides (vancomycin) Beta-lactams3
TMP/SMX Macrolides (azithromycin) Aminoglycosides
Metronidazole Rifampin Tetracyclines (doxy, tigecycline)
Chloramphenicol Ethambutol Clindamycin4
Fosfomycin1 Daptomycin
Isoniazid Colistin
Pyrazinamide Fusion inhibitors (enfurvitide)
Zidovudine Tenofovir
5-flucytosine Amphotericin B5
Voriconazole/fluconazole Echinocandins
Pyrimethamine Itraconazole/posaconazole
Atovaquone?2
Albendazole >>> Praziquantel

1 only FDA-approved for UTI treatment
2 no studies have been published looking at CNS penetration; however has been used successfully in clinical CNS infections
3 overcome by increase in dosages – higher dosages of beta-lactams obtain adequate levels in the CSF and tend to be 1st line agents in bacterial meningitis due to their efficacy and bactericidal properties
4 however has been shown to effectively treat susceptible CNS infections
5 however clinical trials have shown good outcomes when used in treatment of CNS infections

*If the class of drug was not mentioned in this list, it is likely because no studies have been done to assess CNS penetration of that drug.

Why are beta-lactams recommended for empiric
bacterial meningitis treatment?  

Despite the poor CSF penetration, beta-lactams have the most research documenting successful treatment of community-acquired meningitis compared to other antibiotic classes.

  • When the beta-lactam dose is increased, CNS penetration increases.
  • Beta-lactams are well-tolerated even at high dosages
  • Ceftriaxone treats S. pneumoniae, N. meningitidis, H.influenzae, and many aerobic gram-negatives such as E.coli and K. pneumoniae.

*Vancomycin is added to empiric regimens to treat the ceftriaxone-resistant S. pneumoniae strains that have been seen in community-acquired meningitis.

empiric meningitis tx IDSA guidelines                                                            (IDSA practice guidelines for Bacterial Meningitis, 2004.)

TAKE HOME POINTS:

  • Not all antimicrobials penetrate the BBB. Take into account an antimicrobial’s CNS penetration properties when treating CNS infections
  • Beta-lactams are still 1st line therapy for empiric meningitis treatment due to their efficacy against the most common pathogens and ability to achieve high levels with increased doses of the medication
  • When treating CNS infections, deviation from the guidelines warrants involvement of the ID pharmacist and the ID consult team to ensure the best treatment regimen for the patient

References:
1. Bhaskar, S., Tian, F. et al. (2010). Multifunctional Nanocarriers for diagnostics, drug delivery and targeted treatment across blood-brain barrier: Perspectives on tracking and neuroimaging. Particle and fibre toxicology. 7(1)3. DOI: 10.1186/1743-8977-7-3.
2. Nau, R., Sorgel, F, and Eiffert, H. (2010). Penetration of Drugs through the Blood-Cerebrospinal fluid/Blood-Brain Barrier for the treatment of central nervous system infections. Clinical Microbiology Reviews. 23(4): 858-883. DOI: 10.1128/CMR.00007-10
3. Letendre, S. (2011). Central nervous system complications in HIV disease: HIV-associated neurocognitive disorder. Topics in antiviral medicine. 19(4): 137-142.
4. Marra, C. (2014). Central nervous system penetration of ARVs: Does it matter? [powerpoint]. Presented at Northwest Aids Education and Training Center on May 15th, 2014.
5. Cherubin, C.E., Eng, R.H, et al. (1989). Penetration of newer cephalosporins into cerebrospinal fluid. Review of Infectious Diseases.11(4):526-548.
6. Tunkel, A.R., Hartman, B.J, et al. (2004). Practice Guidelines for the management of bacterial meningitis. CID. 39:1267-1284.

Peer-reviewed by Jeffrey Pearson, 2nd year ID pharmacy resident

 

 

Blood culture contaminants

There are some bacteria out there that usually don’t cause disease. They tend to just hang out and not cause any harm. When we see them in the bloodstream, they often are contaminants, meaning they were introduced during collection of the sample, and are not truly in the bloodstream. Figuring out which ones are contaminants and which ones are not can be tricky. Some are almost always (see * below) contaminants while others are NEVER contaminants. Some can be a contaminant or an infection, like streptococcus viridans group.

*Important to note: any organism can cause an infection in the right context, even those who are usually deemed as contaminants. If you are unsure whether a true infection is present, it’s always best to call an infectious disease specialist to assist with management.

Contaminant =  growth of bacteria in the blood culture bottle that were not present in the patient’s bloodstream and thus introduced during the collection of the sample (Dawson et al.)

USUALLY NOT contaminants:

Staphylococcus aureus
Streptococcus pneumoniae
Group A/B streptococcus
Listeria monocytogenes
Neisseria meningitidis
Fungus (yeast or mold)
…Amongst many other numerous organisms that will take too long to mention in a list

Common blood contaminants:

Coagulase-negative staphylococci
Propionibacterium acnes
Micrococcus spp.
Corynebacterium spp.
Bacillus spp. (*except Bacillus anthracis which causes anthrax!!)

What if multiple bacteria are growing?!

  • multiple growth of bacteria can suggest contamination.
  • however, it again depends on the pathogens involved. If all are skin flora, then probably contaminant. If any of the aforementioned ‘USUALLY NOT CONTAMINANTS’ are present, then it is not a contaminant!

 

Where do contaminants come from:

  1. Patient’s skin
  2. Equipment used to obtain sample and transfer it to culture bottle
  3. Provider’s skin
  4. General environment

 

Factors to consider when deciding whether culture
is contaminant or not:

  1. Patient’s clinical status
    • are there any signs or symptoms to suggest infection with this microbe?
    • Example:
      • You would not expect a patient with pneumonia to grow CoNS in their blood.
      • However, you would take CoNS seriously in a patient with recent valve replacement and fevers.
  1. Microbiology of the species – see above
  2. Time to positivity of blood culture – studies have shown that the average time of positivity of true infections is ~12 hours vs. ~24 hours for contaminants
  3. Inoculum of the isolate – how much growth is there?
    • If only 1 out of 4 blood culture bottles are positive –> MORE LIKELY contaminant
    • If >1 out of 4 blood culture bottles are positive –> true pathogen
  1. Any foreign material? – Contaminants become pathogens when they infect hardware and prosthetic valves/grafts. If those are present and there is a possibility they are infected, would not disregard any pathogen as a contaminant.
  2. Patient’s response to antibiotics and isolate’s susceptibility pattern – clues to whether an isolate is a contaminant or a true infection
    • Contaminant: patient does not respond to antibiotics that treat the blood culture isolate
    • True pathogen: patient DOES respond to antibiotics that treat the blood culture isolate
    • Contaminant: patient responds to antibiotics despite blood culture isolate resistant
    • True pathogen: patient does not respond to antibiotics and isolate is resistant

 

FUN FACT: A retrospective review looked at 626 blood cultures and discovered that by 48 hours, 98% of aerobic gram-positive and gram-negative bloodstream infections were identified.

*This shows that unless there is a high suspicion for anaerobe growth, antibiotics can be de-escalated at 48 hours if there is no growth. (Pardo, J., Klinker, K.P, et al. 2014. Time to positivity of blood cultures supports antibiotic de-escalation at 48 hours. Annals of Pharmacology. 48 (1): 33-40).)

TAKE HOME POINTS:

  • Microbes known to be common contaminants CAN cause disease in certain circumstances
  • Always repeat blood culture when first one is positive for microbes
  • If you’re not sure if it’s a contaminant or not ⇒ call ID
    (it’s always better to double check rather than to miss a true infection)

 

References:

1. Dawson, S. 2014. Blood culture contaminants. Journal of Hospital Infection; 87, 1-10. DOI: 10.1016/j.jhin.2014.02.009
2. Pardo, J., Klinker, K.P, et al. 2014. Time to positivity of blood cultures supports antibiotic de-escalation at 48 hours. Annals of Pharmacology. 48 (1): 33-40).
DOI: 10.1177/1060028013511229
3. Hossain, B., Islam, M.S., et al. 2016. Understanding bacterial isolates in blood culture and approaches used to define bacteria as contaminants: a literature review. Pediatric Infectious Disease Journal. 35(5): S45-51.
DOI: 10.1097/INF.0000000000001106
4. Pien, B.C., Sundaram, P., and et al. 2010. The clinical and prognostic importance of positive blood cultures in adults. American Journal of Medicine. 123 (819-828).
DOI: 10.1016/j.amjmed.2010.03.021

Aspiration pneumonia vs. Aspiration pneumonitis

Why this matters:

Let me briefly tell you a story that was published in JAMA:  A patient was admitted to the hospital for seizures and intubated for airway protection. CXR showed infiltrates so patient was started on antibiotics and despite rapid improvement in 24 hours, received a 7 day course of antibiotics ‘just in case’. He was re-admitted to the hospital a week later with severe C.diff infection that did not improve despite adequate treatment and died in the hospital.

 

Antibiotics can cause harm. Sometimes they can be life-saving but risks and benefits need to be weighed each time.

 

Did that patient have aspiration pneumonia or aspiration pneumonitis?

 

Aspiration pneumonia = clinical evidence of pneumonia due to a bacterial infection

Aspiration pneumonitis = chemical lung injury due to gastric acid in the lower airways

– 13-26% can progress to develop bacterial pulmonary superinfections

 

Aspiration pneumonitis Aspiration pneumonia
Fever Low grade only/- +/-
Cough/SOB ++ ++
Hypoxia ++ ++
CXR infiltrate Resolves w/in 48-72 hrs Takes weeks to resolve
Sputum culture negative Positive/negative; purulent
Time to sx resolution Quick (48-72 hrs) Slow (>72 hrs)
Bronchoscopy Bronchial erythema Bronchial purulence

 

Risk factors for aspiration pneumonia:

  1. Risk of aspiration
  • altered mental status
  • esophageal disorders (inc. GERD)
  • neurological disorders that promote dysphagia (i.e. ALS, stroke, etc.)
  • seizures
  • vomiting
  • heavy alcohol use
  • recent intubation, bronchoscopy, upper endoscopy, or NG tube (any mechanical disruption of the natural mechanisms that prevent aspiration)
  1. Risk of aspirating increased inoculum of bacteria
  • poor dental hygiene
  • acid-suppressive medications (H2-blockers, PPIs) – loss of gastric acidity allows more pathogens to survive in the stomach ⇒ higher inoculum is present when aspiration occurs

Prevention of aspiration:

  • aspiration is pretty common in small amounts
  • it becomes pathogenic when a patient has chronic, recurrent aspiration or when the inoculum of bacteria in the aspirate increases to reach the threshold for causing chemical lung injury +/- bacterial infection.
  • there are not enough data to suggest that any interventions help to prevent recurrent aspiration

 

Microbiology:

  1. Flora – oral cavity and stomach (strep spp., H.flu, anaerobes, aerobic GNR)
  2. Community: strep spp., haemophilus influenza > anaerobes
  3. Hospital/long term care facilities:
    staphylococcus aureus, aerobic gram-neg bacilli >> anaerobes
    – these pathogens colonize the oral cavity
    Staph and aerobic GNR are more likely to be the infectious cause in these patients due to their intrinsic higher pathogenicity
  4. Anaerobespeptostreptococcus, fusobacterium nucleatum, Prevotella, Bacteroides spp.
  5. Most of these infections are polymicrobial

 

Diagnosis:

– usually a clinical diagnosis

sputum culture may help to isolate aerobic gram-neg bacilli or staph aureus to alter antimicrobial therapy

– “anaerobic bacteria are virtually never detected in pulmonary infections due to lack of access to specimens that are uncontaminated with the normal flora of the upper airways” (UpToDate) and we also do not culture sputum anaerobically so obligate anaerobes would not be able to grow in a typical sputum culture.

CXR should be ordered to assess for evidence of an infiltrate

  • Imaging: does location of infiltrate matter? YES.
    • aspiration in sitting position: lower lobes
    • aspiration in lying position: lower lobes OR posterior segment of upper lobes

 

Workup:

  • consider repeat CXR to evaluate for resolution of pulmonary infiltrate if patient has clinically improved within 48 hours ⇒ if CXR infiltrate resolved and symptoms resolved, likely pneumonitis and can discontinue the antibiotics.

So far, no trials looking at the utility of pro-calcitonin in differentiating aspiration pneumonitis vs. pneumonia that I know of, although would be a great study to do!

 

When to suspect anaerobic involvement
in pneumonia:

  • indolent course
  • risk factors for aspiration
  • absence of rigors
  • no main isolated pathogen on sputum cultures
  • putrid odor sputum
  • evidence of periodontal disease
  • imaging shows cavitation/necrosis or empyema

 

Treatment:

1. To treat or not to treat

  • It seems difficult to imagine coming into a patient’s room who is in respiratory distress with a leukocytosis and possible fever, and deciding to withhold antibiotics. At that time, everyone will likely start antibiotics.
  • Consider clinically re-evaluation at 48 hours – if CXR infiltrates and symptoms have resolved, it’s likely aspiration pneumonitis and you can probably stop antibiotics

2. Anaerobes: to treat or not to treat

  • community-acquired: always treat for anaerobes in addition to other common pathogens (see above)
  • hospital-acquired: consider treating if patient has poor dentition (that predisposes them to pathogenic anaerobic infections)

3. Antibiotic regimens (with anaerobic coverage)

  1. Community-acquired
    a) Amoxicillin-clavulanate or Ampicillin-sulbactam
    b) Ceftriaxone + metronidazole
    c) Clindamycin (if penicillin/cephalosporin allergic, not ideal regimen due to high resistance rates to certain pathogens)
    d) Moxifloxacin
  2. Hospital-acquired or recent history of antibiotics – cover for drug-resistant pathogens as well
    a) Vancomycin + Piperacillin-tazobactam
    b) Vancomycin + Carbapenem (if history of MDR pathogens)
  3. Duration – 7 days for uncomplicated pneumonia
    *For complicated pneumonia, duration of antibiotics will depend on the complication as well as patient’s clinical status and rate of recovery

 

Complications:

  1. ARDS (can happen both in pneumonitis and pneumonia)
  2. Lung abscess
  3. Empyema

 

Take-home points:

  • Aspiration pneumonitis ≠ Aspiration pneumonia
  • Re-evaluate patient in 48 hours and decide whether patient needs to continue antibiotics
  • Anaerobes are less likely to play a pathogenic role in hospital-acquired pneumonia (consider treatment for anaerobes if patient has poor dentition)

 

References:

1. Bartlett, J. G. 2017. Aspiration pneumonia in adults. Uptodate.
2. Finegold, S.M. 1991. Aspiration Pneumonia. CID. 13(9), S737-S742. DOI: 10.1093/clinids/13.Supplement_9.S737
3. Mandell, L. A. et al. 2007. IDSA guidelines for CAP. Section on aspiration pneumonia. 2007. CID. 44(S2): S27-72. DOI: 10.1086/511159
4. Dragan, V. et al. 2018. Prophylactic antimicrobial therapy for acute aspiration pneumonitis. CID. DOI: 10.1093/cid/ciy120
5. Loeb, M.B. et al. 2003. Interventions to prevent aspiration pneumonia in older adults: a systematic review. Journal of American Geriatric Society. 51(7):1018.
DOI: 10.1046/j.1365-2389.2003.51318.x
6.
Joundi, R.A. et al. 2015. Antibiotics “Just-In-Case” in a Patient with aspiration pneumonitis. JAMA Internal Medicine: Teachable moment – Less is more. 175(4); 489-490. DOI:10.1001/jamainternmed.2014.8030

Bactericidal vs. Bacteriostatic antibiotics

Does it matter whether we use a bactericidal antibiotic or a bacteriostatic one? Surely, the bactericidal one would be more effective, right? The answer is not that simple.

Bactericidal = antibiotics that kill bacteria

Bacteriostatic = antibiotics that inhibit the growth of bacteria (i.e. prevent the bacteria from continuing to grow/proliferate) without killing bacteria in vitro OR it is able to kill the bacteria in vitro but at a slower rate than bactericidal agent does.

– there is thought out there that bacteriostatic agents require more activity from the immune system to eradicate the bacteria (this has not been studied well in literature and there is no data to support or negate this hypothesis).

Bactericidal drugs:                                          Bacteriostatic drugs:
Aminoglycosides                                                Glycylcyclines (tigecycline)
Beta-lactams                                                        Lincosamides (clindamycin)
Fluoroquinolones                                               Macrolides (azithromycin, fidaxomicin)
Glycopeptides (vancomycin)                           Oxazolidinones (linezolid, tedizolid)
Lipopeptides (daptomycin)                             Streptogramins (quinipristin/dalfopristin)
Nitroimidazoles/nitrofurans                           Sulphonamides (sulfamethoxazole)
(metronidazole/nitrofurantoin)                     Tetracyclines
Rifampin

*Keep in mind – bactericidal and bacteriostatic effects depend on several factors, including the amount of:
1) bacterial inoculum: the burden of bacteria can change the antibiotic’s properties
2) the pathogen: certain antibiotics are “cidal” to certain pathogens, while “static” to others
–examples:
a)  vancomycin is “cidal” against staph and strep, but “static” against enterococci
b) azithromycin is “cidal” against strep, but “static” against staph
c) linezolid may be “cidal” against strep, but “static” against staph/enterococci
3) medium/location of infection: certain antibiotics are more effective in certain parts of the body (IV vancomycin doesn’t penetrate GI mucosa, tigecycline doesn’t achieve high concentrations in the bloodstream, etc.)

Why could this be potentially clinically irrelevant?

A systematic review published in CID in 2017 looked at all the 59 randomized-controlled trials on clinical outcomes when using bactericidal vs. bacteriostatic agents. They found that 49 (81%) of the trials showed no differences in clinical outcomes when using “cidal” vs. “static” agents.

Similarly, studies demonstrated that using linezolid (a bacteriostatic agent) against MRSA was non-inferior to vancomycin (a bactericidal agent) against MRSA.
Because clinical outcomes depend on 3 factors:

  1. The host
  2. The pathogen
  3. The drug (with many internal factors coming into play as listed below
    -Tissue penetration
    -Pharmacokinetics
    -Drug interactions
    -Optimal dosing

TAKE-HOME POINTS:

  1. Antibiotics can be bacteriostatic for some pathogens and bactericidal for others
  2. Clinical outcomes depend on a variety of factors and the bactericidal property of an antibiotic ultimately appears to have little clinical relevance.

References:

  • Nemeth, J., Oesch, G., and Kuster, S.P. 2015. Bacteriostatic versus bactericidal antibiotics for patients with serious bacterial infections: systematic review and meta-analysis. Journal of antimicrobial chemotherapy. 70:382-395.
  • French, G.L. 2006. Bactericidal agents in the treatment of MRSA infections – the potential role of daptomycin. Journal of Antimicrobial Chemotherapy. 58 (6): 1107-1117.
  • Wald-Dicker, N, Holtom, P, and Spellberg, B. 2017. Busting the myth of “static vs. cidal”: A systemic literature review. CID
  • Panckey, G.A. and Sabath, L.D. 2004. Clinical relevance og bacteriostatic versus bactericidal mechanisms of action in the treatment of gram-positive bacterial infections. CID, 38(6): 864-870.

 

SPICE organisms

First topic at hand is SPICE organisms. These are the organisms that appear to be sensitive to many antibiotics, but once they are exposed to certain antibiotics (ex. 3rd generation cephalosporins), they quickly develop resistance to them.

SPICE stands for:

S: Serratia spp.

P: Providencia

I: “indole-positive” Proteus spp. (this includes: P. vulgaris) *NOT P.mirabilis

C: Citrobacter spp.

E: Enterobacter spp.

*There are other, less known bacteria included in this group (Cronobacter, Edwardsiella, Hafnia, Morganella, Aeromonas)

 

*[Organisms like Pseudomonas and Acinetobacter produce AmpC gene normally – which is why they have intrinsic resistance to 3rd generation cephalosporins and do not technically fall into the AmpC inducer SPICE group.]

 

The SPICE pathogens can be induced to produce an AmpC beta-lactamase gene that encodes an enzyme that cleaves the beta-lactam group in the antibiotic and renders it inactive.

 

This gene may not be detected initially (low level of expression of the gene) but may appear (induced to express higher levels of gene) after a period of exposure to beta-lactam antibiotics.

(Clinical translation: Initially they will appear susceptible to beta-lactams, but eventually will develop resistance to them. *tricky little bastards, aren’t they?)

 

Once beta-lactam is removed, the AmpC gene production is reduced once more and the pathogens will appear sensitive to 3rd generation cephalosporins and penicillins again. .

 

Resistance develops anywhere from 24h to 2-3 weeks.

 

Clinical relevance:

  • If the course of antibiotics is short or if the antibiotic can easily overcome the MIC concentration needed for bacterial killing, then the risk of inducing AmpC gene production is low
    • Clinical examples (~<1 week duration of antibiotics):
      • UTI
      • Pneumonia
  • Short course for intra-abdominal infectionHowever, this becomes an issue in areas where antibiotics have difficulty penetrating (because it is less likely to overcome the MIC concentration needed) or when antibiotics are needed to be given over a longer period of time.
    • Clinical examples:
      • Endocarditis
      • Bacteremia
      • Osteomyelitis
      • Septic arthritis
      • Abscesses

 

Antibiotics to avoid:

  • Penicillin class (including piperacillin-tazobactam)
  • Most cephalosporins (1st, 2nd, and 3rd generation)

 

Antibiotics to use:

  • 4th generation cephalosporins (i.e. cefepime at higher doses, q8h)
  • Carbapenems
  • Aminoglycosides
  • Fluoroquinolones

TAKE-HOME POINTS:

  1. Remember the members of the SPICE group
  2. You may be successful in treating an infection in short courses of therapy or in infections where antibiotic penetration is high. But in patients with bacteremia, bone, joint, or valve infections – strongly consider 4th generation cephalosporin or a carbapenem.

 

 

Have a question, comment, or a suggestion for a future blog post? Post your comment below!

 

 

References:

  • http://m.antimicrobialstewardship.com/clinical_summaries/index.php?page=esbl_and_spice
  • Jacoby, G.A. AmpC beta-lactamases. Clinical Microbiology Review. 2009. 22(1):161-182. doi: 10.1128/CMR.00036-08
  • Harris, P.N.A, and Ferguson, J.K. Antibiotic therapy for inducible AmpC beta-lactamase-producing Gram-negative bacilli: what are the lternatives to carbapenems, quinolones, and aminoglycosides? 2012. International Journal of Antimicrobial Agents, 40: 297-305.