Citrobacter spp are gram-negative, lactose-positive, aerobic rods of the group of enterobacteriaciae that are colonists of the intestinal tract. The name derives from the fact that the most citrobacteria use citrate as the sole source of carbon.
They are a frequent isolate in urinary tracts infections, sepsis and neonatal meningitis, in frequency ranking below E. coli and Klebsiella spp.
Citrobacter species freundii, amalonaticus and koseri further classified by DNA hybridization (article):
- C. amalonaticus: indole+, ODC+, malonate-
- C. freundii: indole V, ODC-
- C. freundii sensu strictu
- C. youngae
- C. braakii
- C. werkmanii
- C. sedlakii
- C. koseri (diversus): indole+, ODC+, malonate-
Api 20E result for Citrobacter diversus:
Citrobacter spp have inducible, chromosomal AmpC (the C in SPACE) testable with cefoxitin. The presence of AmpC betalactamase confers resistance to penicillins and G1 cephalosporins. Derepression of chromosomal AmpC may lead to resistance to more betalactams including G3 cephalosporins. Prolonged treatment with cephalosporins is therefore not reccomended.
Culture of C. freundii on McConkey agar:
Antibiogram of C. freundii, showing wild type resistance due to inducible AmpC expression. Ampicillin, amoxicillin and clavulanate are inducers of AmpC, resulting in a flattening of zones of other betalactamases near ampicillin and amoxicillin-clavulanate. The inhibition zone of ampicillin 10mg is lager than the inhibition zone of 30 mg amoxicillin+clavulanate due to more AmpC induction by clavulanate:
A antibiogram of C. diversus, flattening of the inhibition zone of ceftazidime due to an inducible betalactamase. Clavulanate restores amoxicillin efficacy and the zone around cefuroxim appears decreased in size as well as the ampicillin zone:
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- Serratia marcescens
- AmpC betalactamase
- Enterobacter aerogenes
- Enterobacter cloacae
- Morganella morganii
- Enterobacteriaciae and betalactams
- Klebsiella pneumoniae
- Proteus vulgaris
- Stenotrophomonas maltophilia
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