Intrinsic antibiotic resistance

Background

• Inherent or innate antimicrobial resistance that is present in all or almost all wild-type isolates of a given species

Enterobacterales

Organism	AMP	AMC	SAM	TIC	CFZ	FOX	CXM	IMI	TET	TGC	NIT	CST	AGs	AZM
Citrobacter freundii	R	R	R		R	R	R							R
Citrobacter koseri	R			R										R
Enterobacter cloacae	R	R	R		R	R								R
Escherichia coli														R
Hadnia alvei	R	R	R		R	R								R
Klebsiella aerogenes	R	R	R		R	R								R
Klebsiella oxytoca	R			R										R
Klebsiella pneumoniae	R			R										R
Morganella morganii	R	R			R		R	±		R	R	R		R
Proteus mirabilis								±	R	R	R	R		R
Proteus vulgaris	R				R		R	±	R	R	R	R		R
Providencia rettgeri	R	R			R			±	R	R	R	R		R
Providencia stuartii	R	R			R			±	R	R	R	R	±	R
Raoultella	R			R										R
Salmonella
Serratia marcescens	R	R	R		R	R	R				R	R		R
Shigella
Yersinia enterocolitica	R	R		R	R									R

• The Morganellaceae may have elevated MICs to imipenem, but should be reported as susceptible if susceptible in vitro
• Providencia stuartii is intrinsically resistant to gentamicin and tobramycin but not to amikacin
• No intrinsic resistance to third-generation cephalosporins, cefepime, aztreonam, ticarcillin-clavulanic acid, piperacillin-tazobactam, and carbapenems
• All are intrinsically resistance to clindamycin, daptomycin, fusidic acid, vancomycin, lipoglycopeptides (vancins and planins), linezolid, tedizolid, quinupristin-dalfopristin, rifampin, macrolides (with the exception of Salmonella and Shigella with azithromycin)

Non-Enterobacterales

Organism	AMP	PIP	TIC	SAM	AMC	TZP	CTX	CRO	CAZ	FEP	ATM	IMI	MEM	ETP	CST	AGs	TET/TGC	TMP	SXT	CHL	FOF
Acinetobacter baumannii	R				R						R			R				R		R	R
Burkholderia cepacia	R	R	R	R	R	±	±	±		±	±	±		R	R	±		±			R
Pseudomonas aeruginosa	R			R	R		R	R						R			R	R	R	R
Stenotrophomonas maltophilia	R	R	R	R	R	R	R	R			R	R	R	R		R	±	R			R

• Stenotrophomonas maltophilia is intrinsically resistant to tetracycline but not to doxycycline, minocycline, or tigecycline
• Non-fermentative Gram-negative bacteria are also intrinsically resistant to penicillin, first- and second-generation cephalosporins, cephamycins, clindamycin, daptomycin, daptomycin, fusidic acid, vancomycin, linezolid, macrolides, quinupristin-dalfopristin, and rifampin

Staphylococci

	NOVO	FOF	FA	AZT	CST	NAL
Staphylococcus aureus				R	R	R
Staphylococcus lugdenensis				R	R	R
Staphylococcus epidermidis				R	R	R
Staphylococcus haemolyticus				R	R	R
Staphylococcus saprophyticus	R	R	R	R	R	R
Staphylococcus capitis		R		R	R	R
Staphylococcus cohnii	R			R	R	R
Staphylococcus xylosus	R			R	R	R

Enterococcus

	Cephalosporins	VAN	TEC	AGs	CLI	Q-D	TMP	SXT	FA
Enterococcus faecalis	R			R	R	R	R	R	R
Enterococcus faecium	R			R	R		R	R	R
Enterococcus gallinarum	R	R		R	R	R	R	R	R
Enterococcus casseliflavus	R	R		R	R	R	R	R	R

• In vitro susceptibility but clinically ineffective: cephalosporins, aminoglycosides, clindamycin, and cotrimoxazole

Anaerobic Gram-Positive Bacilli

	VAN	AGs
Clostridium		R
Clostridioides difficile		R
Clostridium innocuum	R	R

Anaerobic Gram-Negative Bacilli

	AGs	PEN	AMP	FQs
Bacteroides	R	R	R
Fusobacterium canifelinum	R			R

Further Reading

• CLSI M100-ED30:2020 Performance Standards for Antimicrobial Susceptibility Testing, 30th Edition