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Chapter 21: Antimicrobials

An 85-year-old man with a past medical history of COPD and type 2 diabetes presents from a nursing home with acute cholangitis and septic shock. No recent travel history was described. The patient is now on day 3 of empiric treatment with cefepime. Blood cultures are growing Enterobacter cloacae, which is susceptible to gentamicin, ceftriaxone, ceftazidime, cefepime, piperacillin/­tazobactam, meropenem, and ciprofloxacin, and is resistant to cefazolin and cefoxitin. What β-lactamase is most likely produced by the E cloacae isolated in this case?

A. AmpC

B. Klebsiella pneumoniae carbapenemase

C. New Delhi metallo-β-lactamase

D. CTX-M

A. AmpC

Organisms that possess chromosomally encoded inducible AmpC β-lactamase are sometimes referred to as SPACE organisms (Serratia, Providencia, Pseudomonas, Acinetobacter, Citrobacter, Enterobacter). However, other organisms such as Morganella morganii, Proteus vulgaris, and P penneri may be included as well., Since the E cloacae in this case was susceptible to meropenem, possession of a KPC (choice B) or New Delhi metallo-β-lactamase (NDM1; choice C) is unlikely. In addition, the patient does not report recent travel, and NDM1 is associated with healthcare contact in India and Bangladesh. As activity to ceftriaxone and ceftazidime was maintained, then the presence of a CTX-M (choice D), which is a type of extended spectrum β-lactamase, is also less likely.

An 85-year-old man with a past medical history of COPD and type 2 diabetes presents from a nursing home with acute cholangitis and septic shock. No recent travel history was described. The patient is now on day 3 of empiric treatment with cefepime. Blood cultures are growing Enterobacter cloacae, which is susceptible to gentamicin, ceftriaxone, ceftazidime, cefepime, piperacillin/­tazobactam, meropenem, and ciprofloxacin, and is resistant to cefazolin and cefoxitin. What is the most appropriate antimicrobial selection?

A. De-escalate to ceftriaxone

B. Change to piperacillin/tazobactam

C. Continue cefepime

D. Add gentamicin

C. Continue cefepime

Bacteria express 1 of 3 types of AmpC production: (1) low-level constitutive, which refers to a low basal level of production; (2) high-level constitutive, also known as “depressed,” which refers to constant production of high concentrations of AmpC regardless of the presence or absence of an inducer; and (3) inducible production, which refers to low levels of AmpC production initially, but the presence of an inducer causes a surge in production. When the inducer is removed, production returns to basal levels. Β-Lactams differ in their inducing ability and stability against AmpC hydrolysis. Strong inducers ...

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