Case 1:
A 64 year diabetic woman is admitted to the intensive care unit (ICU) in presumed sepsis from a diabetic foot ulcer that probes to bone on exam. She receives surgical debridement; bone is not grossly involved and an amputation is not performed. Intraoperative cultures of the foot wound reveal gram negative rods and gram positive cocci.
In the ICU, she receives pipercillin-tazobactam to cover Pseudomonas aeruginosa and Staphylococcus aureus (methicillin-sensitive) cultured from the wound. She requires ventilator support. On post-operative day 1, her creatinine rises to 6.3 mg / dL and a cardiac troponin comes back at 10 ng / mL. She is diagnosed with an intra-operative myocardial infarct and acute kidney injury. Despite appropriate coverage of the organisms identified from the foot wound, the patient begins to spike fevers (Tmax: 39.2 C) and requires increasing ventilator support. Blood and urine cultures are sent on post-operative day 2.
On post-op day 4, a blood culture is drawn given her persistent fevers despite appropriate antimicrobial therapy. The culture grows gram negative rods. API testing demonstrates this to be Klebsiella pneumoniae. Sensitivity testing reveals the following pattern.
Antibiotic Interpretation
Ampicillin Resistant
Ampicillin-sulbactam Resistant
Cefazolin (1st gen) Resistant
Cefotetatn (2nd gen) Sensitive
Ceftriaxone (3rd gen) Resistant
Ceftazidime (3rd gen) Resistant
Cefepime (4th gen) Resistant
Piperacillin-Tazobactam Sensitive
Gentamycin Resistant
Case 2:
A comatose 36 year old man is admitted to the ICU with
meningeal signs. His lumbar puncture demonstrates slightly blood fluid; opening
pressure is not recorded; normal glucose and protein. CSF is sent for GeneXpert testing and he is
placed on broad-spectrum antibacterial agents to cover bacterial and
mycobacterial meningitis. A malaria
smear is negative.
The ventilated patient is slow to recover spontaneous breathing. His fevers persist, despite broad-spectrum coverage. New cultures from blood, tracheal aspirate, and urine are sent.
Urine, blood cultures (x2), and GeneXpert (CSF) are negative.
The tracheal aspirate grows a single organism on MacConkey's agar. This gram negative is identified by API testing as Klebsiella oxytoca and has the following antibiotic resistance pattern:
Antibiotic InterpretationAmpicillin Resistant
Ampicillin-sulbactam Resistant
Cefazolin (1st gen) Resistant
Cefotetatn (2nd gen) Resistant
Cefoxitin (2nd gen) Sensitive
Ceftriaxone (3rd gen) Resistant
Ceftazidime (3rd gen) Resistant
Cefepime (4th gen) Resistant
Piperacillin-Tazobactam Sensitive
Gentamycin Resistant
The salient features in the patterns of resistance are:
Cephalosporins: Resistant to 1st, 3rd, and 4th generation; Sensitive to 2nd gen
Sensitive to some beta-lactamases (tazobactam > sulbactam)
These patterns are consistent with an extended-spectrum beta-lactamase (ESBL).
Phenotypic testing would demonstrate susceptibility to Clavulonic Acid
ESBLs are important resistance factors to know about and most often are found in Enterobacteriaceae, frequently encountered in Klebsiella spp & Escherichia coli. They are transmissible on large plasmids that often carry additional antibacterial resistance factors; the gentamycin (aminoglycoside) resistance in these vignettes is typical.
ESBLs can be distinguished from another broad-spectrum beta-lactam resistance factor, AmpC, through the following criteria.
1) ESBLs are constitutively expressed, whereas AmpC is induced (the regulation is really cool; more on this later);
2) ESBLs typically test sensitive to 2nd gen cephalosporins but resistant to cefepime (4th gen), whereas AmpC are usually resistant to 1st/2nd/3rd but sensitive to cefepime;
3) ESBLs are often co-transmitted with an additional antibacterial resistance mechanism, thus conferring multidrug (class) resistance; AmpCs are more often genomic, thus not directly associated with a second antibacterial resistance pattern;
AmpC Regulation
The presence of peptidoglycan breakdown products (and, depending upon the bug, other pathways) trigger AmpD to "release the AmpR break" on ampC transcription. AmpC protein is produced, goes to the periplasm, and hydrolyzes the reactive beta-lactam ring (4-Carbon ring in the structure above).
In some organisms, activation of multiple regulatory pathways in sequence leads to a sort of "gear-shifting" through varying levels of AmpC production.
Additional Reading & Acknowledgments:
Many thanks to Al Bateman, PhD and Doug Black, PharmD.
Jacoby, GA. AmpC
Beta-Lactamases. Clin Microbiol Rev.
2009. PMID: 19136439
Geurin F, et al. Complex Regulation Pathways of
AmpC-mediated Beta-lactamase resistance in Enterobacter cloacae complex.
Antimicrob Agents Chemotherp. 2015. PMID: 26438498
Liakopoulos A, et al. A review of SHV Extended-Spectrum
Beta-Lactamases: Neglected Yet Ubiquitous. Front Microbiol. 2016. PMID: 27656166
