Table 1.

Enterobacteriaceae with known β-lactam-resistance mechanisms

Resistance mechanismOrganism (no. of isolates tested)
PrimarySecondary
Chromosomal AmpC β-lactamase, basally expressed Enterobacter cloacae(1)
Chromosomal AmpC β-lactamase, inducibly expressed Enterobacter aerogenes (1)
Enterobacter cloacae (2)
Citrobacter freundii (1)
Morganella morganii (2)
Chromosomal AmpC β-lactamase, constitutively hyperproduced Enterobacter aerogenes (1)
Enterobacter cloacae (2)
Citrobacter freundii (1)
Morganella morganii (2)
Escherichia coli (9)
Chromosomal class A β-lactamase Proteus vulgaris (2)
Kluyvera spp. (2)
Citrobacter diversus (2)
Klebsiella oxytoca (2)
TEM-1 β-lactamase Escherichia coli (4)
Proteus mirabilis (2)
Salmonella spp. (2)
TEM-1 β-lactamase, hyperproduced Escherichia coli (4)
OXA-1 β-lactamase Escherichia coli (1)
Salmonella spp. (1)
SHV-1 β-lactamase Klebsiella pneumoniae (4)
IRT β-lactamase Escherichia coli (3)
Porin-deficient mutant (OmpF) Escherichia coli (3)
Porin-deficient mutant (OmpF)TEM-1 β-lactamase Escherichia coli (4)
Porin-deficient mutant (OmpK36)SHV-1 β-lactamase Klebsiella pneumoniae (1)
Extended spectrum β-lactamasePorin-deficient mutant (OmpF) Escherichia coli (1)
Extended spectrum β-lactamaseChromosomal AmpC β-lactamase, inducibly expressed Enterobacter cloacae (3)
Enterobacter aerogenes (1)
Citrobacter freundii (1)
Extended-spectrum β-lactamaseChromosomal AmpC β-lactamase, constitutively hyperproduced Enterobacter cloacae (1)