Activity of Ertapenem (MK-0826) versus Enterobacteriaceae with Potent β-Lactamases

Abstract

Ertapenem (MK-0826; L-749,345), a new carbapenem with a long serum half-life, was tested, in vitro, against β-lactamase-producing bacteria. The new compound had a MIC at which 90% of the isolates were inhibited of 0.06 μg/ml for extended-spectrum β-lactamase (ESBL)-producing klebsiellas, compared with 0.5 μg/ml for imipenem, 16 μg/ml for cefepime, and >128 μg/ml for ceftazidime and piperacillin-tazobactam. MICs of ertapenem for AmpC-derepressed mutant Enterobacteriaceae were 0.015 to 0.5 μg/ml, whereas imipenem MICs were 0.25 to 1 μg/ml and those of cefepime were 0.5 to 4 μg/ml, and resistance to ceftazidime and piperacillin-tazobactam was generalized. Despite this good activity, the MICs of ertapenem for ESBL-positive klebsiellas mostly were two- to fourfold above those for ESBL-negative strains, and the MICs for AmpC-hyperproducing Enterobacter cloacae and Citrobacter freundii mutants exceeded those for the corresponding AmpC-basal mutants. These differentials did not increase when the inoculum was raised from 10⁴ to 10⁶ CFU/spot, contraindicating significant lability. Carbapenemase producers were also tested. The IMP-1 metallo-β-lactamase conferred substantial ertapenem resistance (MIC, 128 μg/ml) in a porin-deficient Klebsiella pneumoniae strain, whereas a MIC of 6 μg/ml was recorded for its porin-expressing revertant. SME-1 carbapenemase was associated with an ertapenem MIC of 2 μg/ml for Serratia marcescens S6, compared with <0.03 μg/ml for Serratia strains lacking this enzyme. In summary, ertapenem had good activity against strains with potent β-lactamases, except for those with known carbapenemases.

Imipenem and meropenem retain activity against bacteria with extended-spectrum β-lactamases (ESBLs) and those that hyperproduce AmpC type β-lactamases and against Klebsiella oxytoca strains that hyperproduce the K1 enzyme (16). They also retain activity against strains with many less-frequent cephalosporinases, such as the PER and CTX-M enzymes. This activity gives these carbapenem drugs major advantages over cephalosporins, since potent β-lactamases are becoming increasingly prevalent. In recent surveys, ESBLs were found in 23 to 25% of klebsiellas from European intensive care units (2, 19); moreover, Klebsiella strains with multiple ESBLs are increasingly seen (10). AmpC β-lactamases are chromosomal and ubiquitous in Enterobacter spp., Citrobacter freundii, Morganella morganii, and Serratia spp., where they give resistance to oxyimino-aminothiazolyl cephalosporins if they are hyperproduced as a consequence of mutation (16); in addition AmpC genes can escape to plasmids, spreading resistance into further species (20). Hyperproduction of the K1 enzyme is seen in an increasing proportion of K. oxytoca isolates in European intensive care units (2, 11).

The stability of imipenem and meropenem in the presence of potent β-lactamases makes the carbapenems an attractive class for further development (9). Ertapenem (MK-0826, L-749,345) (15) is a new analogue in the class, notable for a long serum half-life (ca. 4 h). We evaluated its activity against strains with potent cephalosporin-hydrolyzing β-lactamases and against Enterobacteriaceae with class A and B β-lactamases that confer resistance to imipenem. The latter enzymes are rare and largely restricted to nonfermenters but may represent a future problem in Enterobacteriaceae too (17, 18).

MATERIALS AND METHODS

Bacteria.

The Klebsiella pneumoniae and K. oxytoca isolates were collected from intensive care unit patients in western and southern Europe during a survey undertaken in 1994 (19). They included 181 klebsiellas inferred to have ESBLs on the basis of ceftazidime MIC/ceftazidime plus clavulanate MIC ratios ≥16. SHV-derived ESBLs in these isolates were typed by isoelectric focusing and PCR-single-strand conformation polymorphism (27); TEM variants were typed by isoelectric focusing and PCR-restriction fragment length polymorphism (26). A few bla_SHV and bla_TEM genes were sequenced (26, 27). Also included, from the same survey (19), were seven Klebsiella isolates with AmpC β-lactamases and 19 K. oxytoca isolates that hyperproduced K1 β-lactamase. Twenty-five K. oxytoca and 25 K. pneumoniae isolates without potent β-lactamase types were included as controls; these were susceptible to extended-spectrum cephalosporins at ≤2 μg/ml and gave ceftazidime MIC/ceftazidime plus clavulanate MIC ratios ≤4.

To assess interactions with inducible and derepressed chromosomal β-lactamases, isogenic mutant series of Enterobacter cloacae, C. freundii, Serratia marcescens, M. morganii, and Proteus vulgaris were tested (3, 25). Most series comprised a β-lactamase-inducible isolate and its β-lactamase-derepressed and -basal mutants, but some lacked β-lactamase-inducible parent strains, having been derived from derepressed isolates. To assess the effect of carbapenemases, the antibiotics were tested against S. marcescens S6, with the SME-1 enzyme (class A) (21, 25), and against K. pneumoniae K4181, a clinical isolate from Singapore with the IMP-1 metallo-β-lactamase and loss of a 39-kDa porin (14). Escherichia coli ATCC 25922 was a general control.

Susceptibility tests.

MICs were determined by National Committee for Clinical Laboratory Standards (NCCLS) agar dilution methodology (22). In some experiments the inocula were adjusted to 10⁴ and 10⁶ CFU/spot to investigate inoculum effects. The drugs tested were ertapenem and imipenem (Merck, Rahway, N.J.), ceftazidime (GlaxoSmithKline, Stevenage, United Kingdom), cefepime (Bristol-Myers Squibb, Hounslow, United Kingdom), and piperacillin and piperacillin-tazobactam (Wyeth, Taplow, United Kingdom). Tazobactam was used at 4 μg/ml. In a few experiments, MICs were redetermined with E-tests (AB Biodisk, Solna, Sweden) on Mueller-Hinton agar, in accordance with the manufacturer's directions.

RESULTS

MICs for klebsiellas in relation to β-lactamase types.

MIC distributions for the klebsiellas with ESBLs, AmpC enzymes, or hyperproduction of the K1 enzyme are illustrated in Table 1. The MIC at which 50% of the isolates were inhibited (MIC₅₀) and MIC₉₀ of ertapenem for the ESBL producers were 0.03 and 0.06 μg/ml, respectively, and were lower than those for any isolate compared, with the new compound about four- to eightfold more active than imipenem and with both these carbapenems considerably more active than any other drug compared. Despite these favorable results, the MICs of ertapenem for ESBL producers were about fourfold above those for nonproducers, which were inhibited by ertapenem at 0.06 μg/ml or less. MICs of ertapenem for three producers were 1 μg/ml or more, although always less than 16 μg/ml. MICs for these three isolates were redetermined with E-tests, which gave values 1 dilution below those found by agar dilution (not shown).

TABLE 1.

MIC distributions for klebsiellas with different β-lactamase profiles

MIC (μg/ml)	No. of isolatesa producing indicated β-lactamase when treated withb:
	Ertapenem					Imipenem					Cefepime
	ESBL	AmpC	K1	None (K pn control)	None (K oxy control)	ESBL	AmpC	K1	None (K pn control)	None (K oxy control)	ESBL	AmpC	K1	None (K pn control)	None (K oxy control)
0.007	8		3	20	1
0.015	49	2	9	4	23
0.03	71	2	3		2									9	18
0.06	39	3	3	1		14	1		1	1	5			13	5
0.12	9		1			125	5	5	17	14	3	2		2	2
0.25						22	1	10	7	8	7	2	6
0.5	2					15		4		2	16	1	3	1
1	1					4					40	1	3
2	1					1					51		4
4											36		3
8	1										13	1
16											8
32											2
64
128
256
>256

No. of isolatesa producing indicated β-lactamase when treated withb:
Ceftazidime					Piperacillin					Piperacillin c-tazobactam
ESBL	AmpC	K1	None (K pn control)	None (K oxy control)	ESBL	AmpC	K1	None (K pn control)	None (K oxy control)	ESBL	AmpC	K1	None (K pn control)	None (K oxy control)
				1
1		6	1	10						1
3	1	3	14	12
4		3	8	2						1
4		4	2										1
2		3								14			10	1
9								3		30		2	9	11
11					1			5	11	58	1	1	4	11
15					3		2	16	10	22			1	1
24					2	1				10	3	2
25	2				6	1				4		14		1
45	2				19	1			2	6	1
38	2				39	1	17		1	7
					111	3			1	28	2

^aValues in boldface and those underlined and in boldface represent numbers of isolates for which the associated MICs are MIC₅₀s and MIC₉₀s, respectively. Where MIC₉₀ = MIC₅₀, the values are given MIC₉₀ labeling. 

^bK pn, K. pneumoniae; K oxy, K. oxytoca. 

Imipenem had a MIC₅₀ of 0.125 μg/ml for both ESBL producers and nonproducers and had a MIC₉₀ only 1 dilution higher for ESBL producers than for nonproducers (0.5 versus 0.25 μg/ml). An imipenem MIC of 1 or 2 μg/ml was recorded for five ESBL producers, whereas MICs for nonproducers were universally 0.5 μg/ml or less. The MICs of the cephalosporins for ESBL producers and nonproducers were much more stratified than were those of the carbapenems (Table 1). Thus, the MIC₅₀ of ceftazidime for ESBL producers was 64 μg/ml, whereas that for nonproducers was 0.25 μg/ml. For cefepime, the MIC₅₀ and MIC₉₀ for ESBL producers were 2 and 8 μg/ml, respectively, compared with 0.06 and 0.12 μg/ml for ESBL-negative K. pneumoniae and 0.03 and 0.06 μg/ml for ESBL-negative K. oxytoca. MICs of piperacillin exceeded 64 μg/ml for the huge majority (93%) of ESBL producers, whereas MICs for most nonproducers were from 4 to 16 μg/ml. Tazobactam, at 4 μg/ml, lowered the MICs of piperacillin for virtually all the isolates, ESBL producing or not. The distribution of MICs of piperacillin-tazobactam for ESBL producers was bimodal, with peaks at 8 and >256 μg/ml and with 69.5% of the producers susceptible at the NCCLS breakpoint (22) of 16 plus 4 μg/ml.

MICs for the 7 klebsiellas with AmpC enzymes are shown in Table 1, as are those for the 19 K. oxytoca isolates that hyperproduced the K1 enzyme. MICs of ertapenem for the AmpC producers were two- to fourfold higher than those for the control klebsiellas but remained ≤0.06 μg/ml, whereas the MICs of imipenem for the AmpC producers were no higher than those for the control strains. Production of AmpC enzymes was associated with high-level resistance (MICs, mostly ≥32 mg/ml) to ceftazidime, piperacillin, and piperacillintazobactam and with MICs of cefepime raised to 0.125 to 0.5 μg/ml, compared with 0.03 to 0.06 μg/ml for control strains.

The MIC₅₀s of both carbapenems were raised by only about 1 dilution for K. oxytoca strains that hyperproduced the K1 β-lactamase: to 0.25 μg/ml for imipenem and 0.015 μg/ml for ertapenem. Hyperproduction of the K1 enzyme was associated with high-level resistance (MICs, mostly ≥64 μg/ml) to piperacillin and piperacillin-tazobactam and with cefepime and ceftazidime MICs raised to 0.25 to 4 μg/ml.

Activity against chromosomal β-lactamase expression mutants.

MICs for AmpC β-lactamase inducibility variants of E. cloacae, C. freundii, M. morganii, and S. marcescens are shown in Table 2, together with data for P. vulgaris mutants varying in expression of their class A chromosomal β-lactamase. Ertapenem MICs for AmpC-derepressed E. cloacae and C. freundii mutants were up to 128-fold above the exquisitely low values for the corresponding AmpC-basal mutants but never exceeded 0.5 μg/ml. AmpC-inducible E. cloacae and C. freundii mostly were more susceptible than their derepressed mutants but less susceptible than their β-lactamase-basal mutants, implying that inducible β-lactamase also gave slight protection against ertapenem. MICs of ertapenem for M. morganii, S. marcescens, and P. vulgaris variants were unrelated to enzyme expression, being equal (±1 dilution) for β-lactamase-inducible, -derepressed, and -basal organisms within a mutant series.

TABLE 2.

MICs for chromosomal β-lactamase inducibility mutant strains of Enterobacteriaceae

Straina	MIC (μg/ml) of:
	Ertapenem	Imipenem	Ceftazidime	Cefepime	Piperacillin	PIPTAZb
C. freundii
C2	0.03	0.5	0.5	0.06	8	4
C2-con	0.125	0.125	64	1	256	64
C2-def	0.007	0.125	0.5	0.06	8	2
C4	0.06	2	8	0.06	16	4
C4-con	0.125	0.25	128	0.5	256	64
C4-def	0.007	0.125	0.06	0.03	4	1
C10	0.007	0.125	0.5	0.03	8	4
C10-con	0.125	0.25	64	0.5	128	32
C10-def	0.004	0.25	0.25	0.015	2	2
C12	0.015	0.5	0.25	0.03	8	4
C12-con	0.25	0.125	64	0.5	128	32
C12-def	0.004	0.125	0.25	0.015	2	2
E. cloacae
84-con	0.25	0.125	128	2	>256	128
84-def	0.015	0.125	2	0.06	16	8
684	0.06	0.125	0.5	0.06	8	4
684-con	0.5	0.25	128	2	256	64
684-def	0.004	0.125	0.25	0.06	4	4
410-con	0.125	0.5	32	0.25	64	16
410-def	0.007	0.125	0.5	0.125	4	4
100-con	0.015	0.125	16	0.125	128	8
100-def	0.007	0.125	0.25	0.03	1	1
M. morganii
M1	0.015	2	0.06	0.015	1	0.25
M1-con	0.015	2	8	0.03	64	0.25
M1-def	0.007	1	0.06	0.015	0.5	0.25
M3-con	0.15	2	16	0.03	64	0.25
M3-def	0.15	4	0.125	0.06	2	0.05
M6	0.015	2	0.06	0.03	1	0.25
M6-con	0.007	2	8	0.015	32	0.25
M6-def	0.007	0.5	0.06	0.015	0.25	0.25
S. marcescens
S2	0.03	1	0.25	0.125	4	4
S2-con	0.06	1	2	0.5	128	64
S2-def	0.015	0.5	0.25	0.125	4	4
S7	0.015	0.5	0.25	0.06	4	2
S7-con	0.03	0.5	1	0.5	128	64
S7-def	0.03	0.5	0.25	0.25	4	4
P. vulgaris
Va1	0.015	1	0.125	0.125	4	1
Va1-con	0.015	0.25	0.125	0.125	32	1
Va1-def	0.007	0.125	0.06	0.06	1	0.25
V2	0.015	1	0.06	0.125	4	1
V2-con	0.007	0.25	0.25	0.5	64	0.5
V2-def	0.007	0.25	0.06	0.06	2	0.5
V3	0.007	1	0.06	0.06	2	1
V3-con	0.007	0.5	0.125	0.5	32	1
V3-def	0.007	0.25	0.06	0.06	0.5	0.5
V11	0.015	0.25	0.03	0.06	0.5	0.5
V11-con	0.015	0.25	0.03	0.06	16	0.5
V11-def	0.007	0.25	0.06	0.06	0.5	0.5
V17	0.015	0.25	0.06	0.06	1	0.5
V17-con	0.007	0.125	0.125	0.5	64	0.5
V17-def	0.004	0.06	0.03	0.03	0.5	0.25

^aSuffix con indicates strains that hyperproduced chromosomal β-lactamases independently of induction; suffix def indicates strains that had only trace levels of chromosomal β-lactamases; strains without suffixes are β-lactamase inducible. 

^bPIPTAZ, piperacillin-tazobactam. 

Imipenem MICs were unrelated to AmpC expression for all species, including E. cloacae and C. freundii. Derepression of AmpC raised the MICs of cefepime by up to 64-fold for E. cloacae and C. freundii, but the MICs for derepressed mutants never exceeded 2 μg/ml; derepression had little effect on the cefepime MICs for S. marcescens, M. morganii, and P. vulgaris. Derepression, but not inducible expression, was associated with resistance to piperacillin and, except in S. marcescens, to ceftazidime. Tazobactam, 4 μg/ml, reversed piperacillin resistance for derepressed P. vulgaris and M. morganii mutants, not for other species.

Inoculum effects.

MICs were determined with inocula of 10⁴ and 10⁶ CFU/spot for selected ESBL-producing klebsiellas and controls and for AmpC-derepressed and -inducible Enterobacteriaceae (Table 3). The maximum inoculum effect with ertapenem for an ESBL producer was eightfold, and most effects were fourfold or less; effects of a similar magnitude were seen for the ESBL-negative klebsiellas used as controls. Inoculum effects with imipenem were slightly greater than those with ertapenem and, again, were unrelated to ESBL production. A much greater differential was seen with cefepime, which typically had 8- to 128-fold inoculum effects for ESBL producers, compared with 2- to 4-fold effects for nonproducers. Most ESBL producers were resistant (MICs, >16 μg/ml) to piperacillin and ceftazidime even at low inocula, precluding calculation of meaningful inoculum effect ratios. The magnitude of the inoculum effects with piperacillin-tazobactam was highly variable. Uniquely among the compounds tested, piperacillin had substantial (8- to 16-fold) inoculum effects for the ESBL-negative klebsiellas, presumably because these have chromosomal LEN, SHV, or K1 enzymes, which are active against piperacillin (16; G. S. Babini and D. M. Livermore, Letter, Antimicrob. Agents Chemother. 44:22302000).

TABLE 3.

Inoculum effects for ertapenem and drugs compared

Isolates (no. tested)	Antibiotic	No. of isolates with:
		Ratio of MICs at inocula of 104 CFU/spot and 106 CFU/spota of:										MIC > 16 μg/ml at inoculum of 104 CFU/spot
		<1	1	2	4	8	16	32	64	128	≥256
Klebsiellas, ESBL producers (50)	Ertapenem		11	25	13	1
	Imipenem		1	11	16	12	9	1
	Ceftazidime		1	4		1						44
	Cefepime				1	8	16	9	10	5		1
	Piperacillin											50
	PIPTAZ			3	1	1	3	2	1	1		38
AmpC-derepressed Enterobacteriaceae	Ertapenem		2	8	5	4
and klebsiellas with plasmid-	Imipenem		3	8	6	1	1
mediated AmpC enzymes (19)c	Ceftazidime				2	2	1					14
	Cefepime			1	7	4	4	3
	Piperacillin											19
	PIPTAZ		1	3	3	1	2	1				8
Klebsiella controls, cephalosporin	Ertapenem		2	5	3
susceptible (10)	Imipenem		2	3	3	1	1
	Ceftazidime	2	1	5	1			1
	Cefepime		1	6	2			1
	Piperacillin				1	3	3	2				1
	PIPTAZ	1	2	5	1							1
AmpC-inducible Enterobacteriaceae	Ertapenem			5	2		2
controls (9)d	Imipenem		1		5	2	1
	Ceftazidime		2	1	2			2	1			1
	Cefepime		2		3	2	1		1
	Piperacillin			3	1		3	1				1
	PIPTAZ		2	2	1	2			1			1

^aRatios were calculated only where the MIC was ≤16 μg/ml with an inoculum of 10⁴ CFU/spot. In some cases the MIC with an inoculum of 10⁶ exceeded the highest drug concentration tested; in these instances the MIC was assumed to be 1 dilution higher. Thus, if the MICs with inocula of 10⁴ and 10⁶ CFU/spot were 8 and >256 μg/ml, respectively, the ratio was scored as 512/8 = 64. 

^bPIPTAZ, piperacillin-tazoloactam. 

^cComprising 4 C. freundii isolates, 3 E. cloacae isolates, 5 K. pneumoniae isolates, 5 M. morganii isolates, and 2 S. marcescens isolates. 

^dComprising 3 C. freundii isolates, 2 E. cloacae isolates, 2 M. morganii isolates, and 2 S. marcescens isolates. 

The inoculum effects of ertapenem and imipenem for AmpC-derepressed strains were 2- to 4-fold, whereas ratios of 8- to 32-fold were seen for cefepime; AmpC-derepressed organisms mostly were resistant to piperacillin and ceftazidime at low inocula, precluding calculation of meaningful inoculum effect ratios. Inoculum effects for the AmpC-inducible control strains were widely scattered for all the compounds; this observation is subject to the caveat that the increased MICs with higher inocula may have reflected the selection of derepressed mutants, not growth of the majority population.

Activity against carbapenemase producers.

K. pneumoniae K4181, with the IMP-1 enzyme and lacking a 39-kDa porin (14), was highly resistant (MIC, >32 μg/ml) to all the β-lactams tested, including both carbapenems (Table 4). The MICs of imipenem and ertapenem fell to 2 and 6 μg/ml, respectively, for a variant that retained the carbapenemase but that had regained expression of the porin. This variant remained highly resistant to the noncarbapenem drugs compared. S. marcescens S6, with the SME-1 enzyme, was resistant to imipenem (MIC, 32 μg/ml) and had reduced susceptibility to ertapenem (MIC, 2 μg/ml) compared with typical S. marcescens strains such as S2 and S7 in Table 2. This strain retained good susceptibility to the other drugs compared.

TABLE 4.

MICs of ertapenem and drugs compared for carbapenemase-producing Enterobacteriaceae

Strain and enzymea	MIC (μg/ml) of:
	Ertapenem	Imipenem	Ceftazidime	Cefepime	Piperacillin	PIPTAZb
K. pneumoniae K4181 IMP-1+ 39-kDa OMP−	>32	>32	>128	>128	>256	>256
K. pneumoniae K4181 IMP-1+ 39-kDa OMP+	6	2	>128	>128	>256	>256
S. marcescens S6 SME-1	2	>32	0.25	0.5	8	4

^aOMP, outer membrane protein. 

^bPIPTAZ, piperacillin-tazobactam. 

DISCUSSION

Established carbapenems have a deserved reputation for activity against ESBL-producing and AmpC-derepressed Enterobacteriaceae, with resistance arising only when these mechanisms are combined with impermeability (5, 16, 17). We examined whether ertapenem shared this favorable behavior and tested its activity against strains with molecular class A and B carbapenemases.

Ertapenem was strongly active against ESBL-producing Klebsiella isolates and AmpC-derepressed Enterobacteriaceae, with MICs mostly ca. 0.03 to 0.12 μg/ml and always ≤4 μg/ml. Nevertheless, ertapenem was ca. fourfold less active against ESBL producers than against nonproducers (Table 1); moreover, E. cloacae and C. freundii strains that hyperproduced AmpC enzymes were less susceptible than their AmpC-basal mutants (Table 2). These data suggest that ertapenem is slightly less stable in the presence of β-lactamase than imipenem and meropenem, which retain full activity against ESBL producers and against AmpC-hyperproducing E. cloacae and C. freundii (2, 3, 16, 25). To investigate this aspect, ESBL-producing and AmpC-derepressed strains were examined for inoculum effects. In general, the effects of β-lactamase lability on MICs become more apparent as the inoculum is raised, as seen here with cefepime against ESBL producers and with piperacillin against control klebsiellas. The inoculum effects observed with ertapenem were small, less even than with imipenem, and substantial lability in the presence of ESBLs and AmpC enzymes could therefore be discounted. The slightly increased MICs of ertapenem for some ESBL producers and AmpC-derepressed strains may reflect other factors. We note by analogy that the MICs of cefoxitin for many ESBL-positive klebsiellas exceed those for ESBL-negative strains (2, 19) but that ESBL-coding plasmids do not raise the MICs of cefoxitin for E. coli transconjugants (13).

AmpC-inducible E. cloacae and C. freundii were more susceptible than their derepressed mutants to ertapenem but less susceptible than the corresponding AmpC- basal mutants. This observation implies that ertapenem, like biapenem and sanfetrinem (3, 6) but unlike imipenem (25), is not a strong inducer of AmpC enzymes in MIC tests. Confirmation of this inference must, however, await direct induction assays.

Acquired carbapenemases are rare but are increasingly being reported, mostly from nonfermenters but occasionally from Enterobacteriaceae. Interest has centered on the IMP and VIM metallo-β-lactamases, which belong to molecular class B, but carbapenemase activity has also been found in a few class A and D enzymes. IMP enzymes are scattered in Pseudomonas aeruginosa and Serratia in Japan and have been found also in isolates from Canada, China, Italy, Hong Kong, and Singapore, (7, 12, 18, 24); VIM enzymes have been found in P. aeruginosa isolates from widely scattered sites across Eurasia (18). Class D β-lactamases with carbapenemase activity have been found in Acinetobacter spp. worldwide but not in other genera (1, 4, 8); class A β-lactamases able to hydrolyze carbapenems have been found in tiny numbers of Serratia and Enterobacter spp. from Europe and North America (18). K. pneumoniae K4181 with IMP-1 β-lactamase and lacking a 39-kDa porin was resistant to all β-lactams tested, including ertapenem. Resistance to carbapenems, but not to other antibiotics, was reduced when expression of the porin was restored. The view that IMP enzymes require impermeability (or some other factor) to confer carbapenem resistance is supported by the observations that imipenem MICs for E. coli transconjugants with the IMP-1 enzyme are only ca. 2 μg/ml (14, 23) and that many bla_IMP⁺P. aeruginosa isolates express resistance to ceftazidime but not to imipenem (24). S. marcescens S6, with the SME-1 enzyme (21), was resistant to imipenem and had reduced susceptibility to ertapenem (MIC, 2 μg/ml) as well as to meropenem (MIC, 2 μg/ml [3, 21]). It is unclear whether these MICs equate to clinical resistance; they are ca. 32-fold higher than those for typical S. marcescens strains but still below the NCCLS breakpoints for imipenem and meropenem, which are as follows: susceptible, ≤4 μg/ml; resistant, ≥16 μg/ml (22). Provisional NCCLS MIC breakpoints for ertapenem are identical (NCCLS summary minutes of the meeting of the Subcommittee on Antimicrobial Susceptibility Testing, 7 to 9 June 1998, p. 15–16) but await formal confirmation and ratification.

In summary, ertapenem has acceptable stability in the presence of AmpC and ESBLs, which are increasingly widespread in current isolates. In the future, ertapenem and other carbapenems may be threatened by the spread of IMP-1 and other metallo-β-lactamases. The magnitude of this threat is likely to depend on the level of usage, and suitable surveillance in warranted. Spread of SME-1-like enzymes seems a lesser hazard, as none has ever been shown to be transferable among bacteria.