Table 1.

β-lactam resistance mechanisms in bacteria associated with cIAI

Organisms	Common β-lactam resistance determinants	Antibiotic resistance phenotype
Community-associated	CTX-M Class A ESBLs	Penicillins, extended-spectrum cephalosporins
Enterobacteriaceae	KPC, SME Class A carbapenemases Metallo-β-lactamases (eg, NDM, IMP, VIM types)	Penicillins, cephalosporins, carbapenems
Hospital-associated Enterobacteriaceae	TEM, SHV Class A ESBLs as well as those in community	Penicillins, extended-spectrum cephalosporins
	KPC, SME Class A carbapenemases Metallo-β-lactamases (eg, NDM, IMP, VIM types) Derepressed AmpC β-lactamases (eg, P99 or CMY-2 in Enterobacter spp.); Other AmpC and extended spectrum AmpC (ESAC) β-lactamases eg, CMY Class C; β-lactamases	Penicillins, cephalosporins, carbapenems Penicillins, cephalosporins including cephamycins; carbapenems in OprD downregulated bacteria Cephalosporins, including cephamycins; ESACs may be more susceptible to tazobactam
Pseudomonas aeruginosa	Downregulation of OprD Efflux pumps (MexAB/OprM) Chromosomal AmpC of P. aeruginosa Acquired ESBLs (TEM, SHV, OXA) Acquired carbapenemases (KPC, OXA, metallo-β-lactamases like NDM, VIM, IMP, SPM types)	Carbapenems, some cephalosporins, penicillins Meropenem, some cephalosporins, penicillins Anti P. aeruginosa Penicillins, anti P. aeruginosa Cephalosporins except ceftolozane Anti P. aeruginosa Cephalosporins except ceftolozane, cefepime Anti P. aeruginosa Penicillins, cephalosporins and carbapenems
Acinetobacter spp.	Chromosomal Class C β-lactamases (ADC types) Acquired ESBLs Acquired carbapenemases (OXAs) Porin deletions Rnd-type efflux mechanisms	Penicillins, cephalosporins, including cephamycins Penicillins, extended-spectrum cephalosporins Carbapenems Penicillins, cephalosporins, carbapenems Imipenem, tigecycline
Enterococci (E. faecalis)	Altered PBPs (PBP5) Low-affinity PBPs	Ampicillin, piperacillin Intrinsic resistance to all cephalosporins and carbapenems
Staphylococcus aureus	Class A β-lactamase, PC1 Altered PBPs-PBP2a	Penicillin Resistance to all β-lactams except ceftaroline fosamil

Abbreviations: cIAI, complicated intra-abdominal infections; ESBLs, extended-spectrum β-lactamases; KPC, Klebsiella pneumoniae carbapenemase; CTX-M, cefotaximase-Munich; SME, Serratia marcescens enzyme; TEM, chromosomal β-lactamase of E. coli; SHV, sulfhydryl variant of TEM; CMY-2, cephamycinase; AmpC, class C β-lactamase; OprD, outer porin D; Mex AB, multidrug efflux pumps AB of pseudomonas; OprM, outer porin M; OXA, oxacillinase; SPM, Sao Paulo metallo-β-lactamase; ADC, Acinetobacter derived cephalosporinase; Rnd, resistance nodulation division; PBP, penicillin binding protein; PC1, class A β-lactamase of S. aureus.