Gram Negatives

A 1939 conceptual illustration how Salmonella enterica serovar Typhi may contaminate clean water supplies.

Image from: "ForskeligeVeje ad hvilkenBroen kan inficeres medTyfusbaciller.png." Wikimedia Commons. Uploaded April 15, 2007. Accessed June 30, 2023. https://commons.wikimedia.org/wiki/File:ForskeligeVeje_ad_hvilkenBroen_kan_inficeres_medTyfusbaciller.png

1. Name the ESKAPE pathogens.

Answer below.

Enterococcus faecium, Staphylococcus aureus, Klebsiella pneumoniae, Acinetobacter baumannii, Pseudomonas aeruginosa, Enterobacter spp.

De Oliveira DMP, Forde BM, Kidd TJ, Harris PNA, Schembri MA, Beatson SA, Paterson DL, Walker MJ. Antimicrobial Resistance in ESKAPE Pathogens. Clin Microbiol Rev. 2020 May 13;33(3):e00181-19.

2. Name the HACEK organisms.

Answer below.

Haemophilus, Aggregatibacter, Cardiobacter hominis, Eikenella corrodens, and Kingella are all fastidious, Gram-negative bacteria that colonize the oropharynx and are recognized as a cause of infective endocarditis.

Chambers ST, Murdoch D, Morris A, Holland D, Pappas P, Almela M, Fernández-Hidalgo N, Almirante B, Bouza E, Forno D, del Rio A, Hannan MM, Harkness J, Kanafani ZA, Lalani T, Lang S, Raymond N, Read K, Vinogradova T, Woods CW, Wray D, Corey GR, Chu VH; International Collaboration on Endocarditis Prospective Cohort Study Investigators. HACEK infective endocarditis: characteristics and outcomes from a large, multi-national cohort. PLoS One. 2013 May 17;8(5):e63181.

3. Name the aerobic, lactose-fermenting Gram-negative bacilli of clinical significance.

Answer below.

Escherichia coli, Klebsiella spp, Citrobacter spp, Enterobacter spp, Serratia spp

Jung B, Hoilat GJ. MacConkey Medium. [Updated 2022 Sep 26]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2023 Jan-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK557394/

4. What are the preferred antibiotics for treatment of uncomplicated cystitis caused by extended beta-lactamase-producing enterobacterales (ESBL-E)?

Answer below.

Nitrofurantoin and TMP-SMX are preferred agents for treatment of uncomplicated cystitis caused by ESBL-E organisms. Ciprofloxacin, levofloxacin, and carbapenems arealternative agents for uncomplicated cystitis caused by ESBL-E organisms.

Tamma PD, Aitken SL, Bonomo RA, Mathers AJ, van Duin D, Clancy CJ. IDSA 2023 Guidance on the Treatment of Antimicrobial Resistant Gram-Negative Infections. https://www.idsociety.org/practice-guideline/amr-guidance/ Last Updated June 7, 2023. Accessed June 14, 2023.

5. What are the preferred antibiotics for treatment of complicated cystitis and pyelonephritis caused by extended beta-lactamase-producing enterobacterales (ESBL-E)?

Answer below.

TMP-SMX, ciprofloxacin, and levofloxacin are preferred agents for treatment of complicated cystitis and pyelonephritis caused by ESBL-E organisms. Ertapenem, meropenem, and imipenem-cilastatin may also be used if resistance or other contraindications preclude the use of the preferred agents. Aminoglycosides may also be considered as an alternative based on toxicities or resistance patterns.

6. What are the preferred antibiotics for treatment of infections due to extended beta-lactamase-producing enterobacterales (ESBL-E) outside of the urinary tract?

Answer below.

Ertapenem, meropenem, and imipenem-cilastatin are preferred agents. After appropriate clinical response is achieved and the patient is no longer critically ill, TMP-SMX, ciprofloxacin, and levofloxacin may be considered for oral transition, as long as susceptibility is demonstrated.

7. Which Gram-negative organisms of clinical significance are likeliest to harbor ESBL enzymes?

Answer below.

Escherichia coli, Klebsiella pneumoniae, Klebsiella oxytoca, and Proteus mirabilis.

8. Which beta-lactamase enzyme is most commonly found mediating resistance among ESBL enterobacterales in the United States?

Answer below.

CTX-M enzymes. In particular, CTX-M-15 is the most common extended spectrum beta-lactamase currently found in the United States.

9. ESBL testing is usually not performed in most clinical microbiology laboratories; but instead, non-susceptibility to which antibiotic is commonly used as a proxy for the presence of an ESBL?

Answer below.

Non-susceptibility to ceftriaxone is often used as a proxy for testing the presence or absence of an ESBL in Gram-negative organisms. This is obviously non-specific, and sometimes resistance to ceftriaxone can occur for other reasons.

10. Why is piperacillin-tazobactam an unreliable choice in treating ESBL-producing infections?

Answer below.

There is a risk of clinical failure, even if in vitro susceptibilities suggest it may be effective. From the IDSA: " First, piperacillin-tazobactam MIC testing may be inaccurate and/or poorly reproducible when ESBL enzymes are present, or in the presence of other β-lactamase enzymes such as OXA-1, making it unclear if an isolate that tests susceptible to this agent is indeed susceptible... Second, in vitro data indicate that with increased bacterial inoculum (e.g., abscesses), piperacillin-tazobactam may no longer be effective against ESBL-E when compared to meropenem... Additionally, the effectiveness of tazobactam may be diminished by organisms with increased expression of ESBL enzymes or by the presence of multiple ESBL or other β-lactamases... Finally, there are ESBL enzymes that are inhibitor resistant (i.e., not inhibited by β-lactamase inhibitors)."

11. Is there a similar concern for using cefepime to treat ESBL-producing infections?

Answer below.

Again, there is a risk of clinical failure, even if in vitro susceptibilities suggest it may be effective. From the IDSA: "ESBLs commonly hydrolyze cefepime... Furthermore, even if ESBL-producing isolates test susceptible to cefepime, cefepime MIC testing may be inaccurate and/or poorly reproducible with commercial AST methods... Clinical trials designed to compare the outcomes of patients with ESBL-E bloodstream infections treated with cefepime or carbapenem have not been conducted. If cefepime was initiated as empiric therapy for uncomplicated cystitis caused by an organism later identified as an ESBL-E and clinical improvement occurs, no change or extension of antibiotic therapy is necessary, as uncomplicated cystitis often resolves on its own. Limited data are available evaluating the role of cefepime versus carbapenems for ESBL-E pyelonephritis and cUTIs... A clinical trial evaluating the treatment of molecularly confirmed ESBL-E pyelonephritis and cUTI was terminated early because of a high clinical failure signal with cefepime (2 g IV every 12 hours), despite all isolates having cefepime MICs of 1-2 µg/mL... It is unknown if results would have been more favorable with every 8-hour cefepime dosing. Until larger, more robust comparative effectiveness studies are available to inform the role of cefepime, the panel suggests avoiding cefepime for the treatment of ESBL-E pyelonephritis or cUTI."

12. Which enterobacterales species should be considered highest risk for AmpC beta-lactamase production due to an inducible ampC gene?

Answer below.

Enterobacter cloacae complex, Klebsiella aerogenes, and Citrobacter freundii.

13. Which antibiotics are likely to induce ampC genes?

Answer below.

Many beta-lactam antibiotics may induce AmpC production, but the strongest inducers are aminopenicillins, first-generation cephalosporins, and cephamycins. Imipenem is also a strong inducer, although it is resistance to AmpC hydrolysis. Piperacillin-tazobactam, ceftriaxone, ceftazidime, and aztreonam are relatively weak inducers, but unfortunately they are susceptible to AmpC hydrolysis, so they can be risky choices for treatment of infection due to AmpC-producing organisms.

14. What are the most commonly encountered carbapenemases in the United States?

Answer below.

K pneumoniae carbapenemases, which are not limited to K pneumoniae isolates.

15. A 4-year-old child with short gut and TPN-dependent status with a central catheter in place is admitted for fever. Within 24 hours, the child's blood culture demonstrates growth with Enterobacter cloacae. The primary team removes the child's central catheter. It is noted on subsequent susceptibility testing that the Enterobacter isolate is susceptible to second-, third-, and fourth-generation cephalosporins. The primary team asks which antibiotic would be preferred to complete a course of treatment after the bacteremia resolves.

A. Cefoxitin, in order to avoid induction of an extended-spectrum beta-lactamase

B. Ceftriaxone, for ease of dosing

C. Cefepime, in order to avoid induction of an ampC beta-lactamase

D. Meropenem

Answer below.

C. Cefepime, in order to avoid induction of an ampC beta-lactamase. Note that cefoxitin and ceftriaxone may induce ampC beta-lactamase production. Meropenem could reasonably work, although this provides excessively broad Gram-negative coverage when the isolate is already demonstrated to be cefepime-susceptible. "E. cloacae, E. aerogenes, and most strains of Chronobacter are intrinsically resistant to ampicillin and first- and second-generation cephalosporins as a result of an inducible AmpC chromosomal β-lactamase that is controlled by both positive and negative regulators... Clinicians must be aware that emergence of stably derepressed resistant mutants may lead to treatment failure when third-generation cephalosporins are used, even if isolates appear susceptible at initial testing. Therefore, fourth-generation cephalosporins and carbapenems may be empirically chosen for severe infections."

Nelson GE, Green MH. "Enterobacteriaciae," in: Mandell, Dogulas, and Bennett's Principles and Practice of Infectious Diseases. Bennett JE, Dolin R, Blaser MJ, eds. 9th edition. Elsevier; 2020: 2669-2685.

16. Name every available drug in the United States that could be effective against a pan-susceptible Pseudomonas aeruginosa isolate, notwithstanding privileged anatomic compartments (regardless of CNS infection, etc).

Answer below.

Piperacillin-tazobactam, ceftazidime, cefepime, imipenem, meropenem, aztreonam, ciprofloxacin, levofloxacin, tobramycin, gentamicin, ceftolozane-tazobactam, ceftazidime-avibactam, meropenem-vaborbactam, imipenem-cilastatin-relebactam, cefidericol.

Gilbert DN, Chambers HF, Saag MS, Pavia AT, Boucher HW, Black D, Freedman DO, Kim K, Schwartz BS, eds. The Sanford Guide to Antimicrobial Therapy 2023. 53rd ed. Sperryville, VA: Antimicrobial Therapy, Inc. 2023.

17. Name 4 species of enterobacterales associated with brain abscesses in infants.

Answer below.

Citrobacter koseri, Cronobacter sakazakii, Serratia marcescens, and Salmonella species are associated with increased risk for brain abscesses in infants with meningitis caused by these organisms.

American Academy of Pediatrics. Serious Neonatal Bacterial Infections Caused by Enterobacteriaceae. In: Kimberlin DW, Barnett ED, Lynfield R, Sawyer MH, eds. Red Book: 2021 Report of the Committee on Infectious Diseases. Itasca, IL: American Academy of Pediatrics. 2021: 311-315

18. A 3-year-old child who has a tracheostomy and is ventilator dependent develops pneumonia. Respiratory cultures demonstrate the presence of 4+ Stenotrophomonas maltophilia. Which of the following is the drug of choice for treating this child's infection?

A. Levofloxacin

B. Cefepime

C. TMP-SMX

D. Meropenem

Answer below.

C. TMP-SMX. This drug remains the mainstay of therapy, as it maintains the greatest in vitro potency against clinical isolates. Quinolones are sometimes used as alternative therapy, but resistance to this class is growing. S maltophilia is intrinsically resistant to most beta-lactam antibiotics.

Greenberg D. "Stenotrophomonas maltophilia and Burkholderia cepacia complex," in: Mandell, Dogulas, and Bennett's Principles and Practice of Infectious Diseases. Bennett JE, Dolin R, Blaser MJ, eds. 9th edition. Elsevier; 2020: 2700-2705.

19. How many species of Salmonella are there? How many subspecies? How many serovars?

Answer below.

Currently only 2 species are recognized: S enterica and S bongori. There are 6 subspecies among S enterica, including S enterica subsp enterica, which comprises most human infections. There are more than 2600 serovars described.

American Academy of Pediatrics. Salmonella Infections. In: Kimberlin DW, Barnett ED, Lynfield R, Sawyer MH, eds. Red Book: 2021 Report of the Committee on Infectious Diseases. Itasca, IL: American Academy of Pediatrics. 2021: 655-653

20. What is the sensitivity of blood cultures in children with enteric fever? What about bone marrow cultures?

Answer below.

Blood cultures are estimated to be 60% sensitive in enteric fever. Bone marrow cultures are estimated to be 90% sensitive.

21. How long does non-typhoidal Salmonella meningitis require treatment?

Answer below.

Salmonella meningitis should be treated at least 4 weeks.

22. What should be done for anybody who accompanied an index case of typhoid while travelling?

Answer below.

All people who traveled with the index case should have a stool culture performed. If results are positive, treatment should be initiated with azithromycin or a fluoroquinolone and the patient should be monitored for development of symptoms.

23. What percentage of patients with non-typhoidal Salmonella bacteremia may have a focal infection?

Answer below.

Up to 10%. This is especially true for infants and immunocompromised individuals.

24. How long can children infected with non-typhoidal Salmonella shed viable organisms in their stool?

Answer below.

Approximately 45% of children 5 years old and younger will excrete Salmonella organism for up to 12 weeks.

Only 5% of older childre and adults will experience prolonged shedding for up to 12 weeks.

25. Which Salmonella serovars are the most common to be isolated from blood and cerebrospinal fluid (and thus represent the more common invasive serovars)?

Answer below.

Typhimurium, Enteritidis, I:4, [5], 12:i:-, and Dublin.

26. All patients with meningococcal disease should be screened for which immune deficiency?

Answer below.

Complement component deficiencies. In some studies, complement deficiencies have been found in 10-50% of patients with meningococcal disease.

American Academy of Pediatrics. Meningococcal Infections. In: Kimberlin DW, Barnett ED, Lynfield R, Sawyer MH, eds. Red Book: 2021 Report of the Committee on Infectious Diseases. Itasca, IL: American Academy of Pediatrics. 2021: 519-532

27. Which meningococcal serogroups cause 85% of disease in adolescents?

Answer below.

B, C, Y, W

28. Which 3 chemoprophylaxis regimens may be used for high risk contacts of patients with meningococcal disease?

Answer below.

Rifampin, ceftriaxone, ciprofloxacin. Azithromycin is also an option, altough it is not recommended routinely due to a lack of data.

29. What kind of high risk contacts warrant chemoprophylaxis among individuals exposed to a case of meningococcus?

Answer below.

High risk situations which warrant chemoprophylaxis:

- Household contacts

- Child care or preschool contact any time during the 7 days prior to onset of illness

- Direct exposure to the index patient's secretions any time within 7 days prior to onset of illness

- Mouth-to-mouth rescuscitation any time from 7 days prior to onset of illness to 24 hours after initiation of antibiotics

- Sleeping in the same dwelling as the index case during 7 days prior to onset of illness

- Passengers seated directly next to the index case on flights lasting more than 8 hours, or passengers seated within one seat of the index case on flights of any duration if they were actively cough or vomiting during flight

Low risk situations which do not warrant chemoprophylaxis:

- Casual contact with no history of direct exposure to oral secretions (such as school or work)

- Health care personnel without direct exposure to oral secretions

30. Who is at greatest risk of invasive disease with Haemophilus influenzae type b (Hib) infection?

Answer below.

Unimmunized children younger than 5 years; sickle cell disease patients; asplenic patients; HIV patients; and those with immunodeficiency syndromes and those undergoing chemotherapy. In addition, Hib infections have been noted to be historically more common among Black and American Indian/Alaska Native children, boys, child care attendees, those in crowded conditions, and non-breastfed children.

American Academy of Pediatrics. Haemophilus influenzae Infections. In: Kimberlin DW, Barnett ED, Lynfield R, Sawyer MH, eds. Red Book: 2021 Report of the Committee on Infectious Diseases. Itasca, IL: American Academy of Pediatrics. 2021: 345-354

31. A 2-year-old unimmunized boy presents to your facility with a 2-day history of fever, vomiting, increasing lethargy followed by unresponsiveness and seizure. Blood, CSF, and urine cultures are obtained. His CSF chemistries reveal pleocytosis of 1,000 WBCs with a neutrophilic predominance. His blood and CSF cultures demonstrate growth of Haemophilus influenzae type b (Hib). Of note, he lives in a household with 2 other siblings, a 6-year-old who is fully immunized, and a 6-month-old who is unimmunized. They also have a pregnant aunt staying in their home. The boy also attends daycare. Which of the following contacts warrants chemoprophylaxis?

A. Only the 6-month-old younger sibling

B. All members of the household

C. All members of the household and his daycare contacts

D. The pregnant aunt and the 6-month-old sibling

Answer below.

Answer: B All members of the household.

Chemoprophylaxis is recommended for ALL HOUSEHOLD CONTACTS in the following circumstances: there is at least 1 child younger than 4 years who is unimmunized or incompletely immunized; there is at least 1 child younger than 12 months who has not completed the primary Hib series; there is an immunocompromised child, regardless of Hib immunization status or age. As for preschool and child care contacts, chemoprophylaxis is only recommended when 2 or more cases of Hib invasive disease have occurred within 60 days. Chemoprophylaxis is not recommended on the basis of pregnancy alone, but the aunt's cohabitation in a household with the unimmunized infant warrants treatment.

Chemoprophylaxis is intended to eliminate asymptomatic carriage in individuals who could place vulnerable household contacts at risk (such as immunocompromised or unimmunized small children). Rifampin should be administered orally once daily for 4 days; note that rifampin is contraindicated in pregnancy, and ceftriaxone may be used instead, although there is less data behind this recommendation.

32. In what season do the majority of cases of Burkholderia pseudomallei infection occur?

Answer below.

75% of cases of melioidosis occur in the rainy season.

American Academy of Pediatrics. Burkholderia Infections. In: Kimberlin DW, Barnett ED, Lynfield R, Sawyer MH, eds. Red Book: 2021 Report of the Committee on Infectious Diseases. Itasca, IL: American Academy of Pediatrics. 2021: 240-243

33. What percentage of adults with melioidosis are bacteremic upon admission?

Answer below.

50%

34. Which antibiotic is most active against the majority of Burkholderia cepacia isolates?

Answer below.

Meropenem and possibly ceftazidime. While potentially active drugs include TMP-SMX, ceftazidime, minocycline, and flouroquinolones, resistance is common. Treatment of severe or invasive infections should include meropenem or ceftazidime initially.

35. What are NICU outbreaks of Elizabethkingia meningoseptica linked to?

Answer below.

Elizabethkingia meningoseptica infections in intensive care units can be quite destructive. Mortality rates 50% and greater have been recorded. Outbreaks have been associated with respiratory equipment, contaminated sinks, infusion containers, saline for eye rinses of newborns, and rubber stoppers for milk bottles.

Jean SS, Lee WS, Chen FL, Ou TY, Hsueh PR. Elizabethkingia meningoseptica: an important emerging pathogen causing healthcare-associated infections. J Hosp Infect. 2014 Apr;86(4):244-9.

36. What makes Elizabethkingia meningoseptica susceptibility profiles so odd?

Answer below.

Elizabethkingia meningoseptica may be susceptible to antibiotics more commonly utilized against Gram-positive organisms, like vancomycin and linezolid. They can often be highly resistant to more traditional antibiotic choices for Gram-negatives, like third and fourth-generation cephalosporins and carbapenems.

Jean SS, Lee WS, Chen FL, Ou TY, Hsueh PR. Elizabethkingia meningoseptica: an important emerging pathogen causing healthcare-associated infections. J Hosp Infect. 2014 Apr;86(4):244-9.

37. Describe Edwardsiella tarda. Where is it found?

Answer below.

Edwardsiella tarda is an anaerobic, facultative motile, Gram-negative, lactose-nonfermenting member of the family enterobacterales. It can be mistaken for Salmonella. It is found in warm, brackism tropical and subtropical waters in Japan, China, the Gulf of Mexico, and the United States. It is usually carried by marine animals and amphibians. Upon ingestion of contaminated seafood, it may lead to gastroenteritis and severe, invasive infections in the immunocompromised.

Healey KD, Rifai SM, Rifai AO, Edmond M, Baker DS, Rifai K. Edwardsiella tarda: A Classic Presentation of a Rare Fatal Infection, with Possible New Background Risk Factors. Am J Case Rep. 2021 Dec 7;22:e934347.

38. What mediator of resistance is found in almost all species of Moraxella?

Answer below.

Almost all strains produce beta-lactamase and are resistant to amoxicillin.

American Academy of Pediatrics. Moraxella catarrhalis Infections. In: Kimberlin DW, Barnett ED, Lynfield R, Sawyer MH, eds. Red Book: 2021 Report of the Committee on Infectious Diseases. Itasca, IL: American Academy of Pediatrics. 2021: 537-538

39. Describe the microscopic morphology of Yersinia enterocolitica.

Answer below.

Non-spore-forming, pleomorphic, bipolar-staining ("safety-pin"), Gram-negative coccobacilli.

Woods CR. "Other Yersinia Species," in: Cherry JD, ed. Feigin and Cherry's Textbook of Pediatric Infectious Diseases. 8th edition. Elsevier; 2019: 1085-1097.

40. Who is at increased risk of septicemia due to Yersinia enterocolitica?

Answer below.

Young infants, those with immunodeficiencies or immunodeficient states, diabetes mellitus, and those with iron overload states, such as those receiving multiple transfusions for hematologic diseases (sickle cell, thalassemias), or due to other causes of excess serum iron (hemochromatosis, cirrhosis). Deferoxamine therapy is also a risk factor, as the Yersinia bacterium is able to extract iron from this compound.

Woods CR. "Other Yersinia Species," in: Cherry JD, ed. Feigin and Cherry's Textbook of Pediatric Infectious Diseases. 8th edition. Elsevier; 2019: 1085-1097.