Erythromycin and Clindamycin Resistance and Telithromycin Susceptibility in Streptococcus agalactiae

Abstract

The rates of resistance to erythromycin and clindamycin among Streptococcus agalactiae strains isolated in our hospital increased from 4.2 and 0.8% in 1993 to 17.4 and 12.1%, respectively, in 2001. Erythromycin resistance was mainly due to the presence of an Erm(B) methylase, while the M phenotype was detected in 3.8% of the strains. Telithromycin was very active against erythromycin-resistant strains, irrespective of their mechanisms of macrolide resistance.

Streptococcus agalactiae is a well-known cause of invasive infections in neonates and pregnant women. It has increasingly been recognized as a significant pathogen in nonpregnant adults, especially among patients with underlying conditions (19). Penicillin and ampicillin are the drugs of choice for prevention or treatment of S. agalactiae infections, and clindamycin and erythromycin are the recommended alternatives for patients who are allergic to β-lactam agents. In the majority of susceptibility studies (1, 7, 11-14, 16, 20) penicillin remains uniformly active against S. agalactiae, although there are scattered reports of nonsusceptibility to penicillin or ampicillin (6, 9, 21). In 1992, our working group detected penicillin-intermediate isolates (3), but at a very low incidence, and since then we have not found similar strains. Resistance of S. agalactiae to erythromycin and clindamycin has increased during the last decade in several countries with some geographical variations. In Taiwan, Hsueh et al. (9) reported an increase in erythromycin and clindamycin resistance from 19 and 18% in 1994 to 46 and 37% in 1997, respectively. In the United States, Morales et al. (13) found that rates of erythromycin resistance rose from 1.2% among the isolates collected in 1980 to 1993 to 18% during 1997 and 1998; the increase appeared to be related to an increase in macrolide usage. Most studies (1, 6-9, 12, 14, 16) reported rates of resistance to erythromycin higher than those for clindamycin. In contrast, Ko et al. (11) and Uh et al. (20) have found resistance to clindamycin to be more common than resistance to erythromycin.

The increasing trend in the rates of resistance to erythromycin and clindamycin among S. agalactiae isolates has raised concerns about the use of these antibiotics as alternative agents for the prophylaxis or treatment of S. agalactiae infections. Telithromycin belongs to a new semisynthetic 14-membered-ring macrolide class of antibiotics, the ketolides.

This study had three objectives: (i) to examine the evolution of susceptibility to erythromycin and clindamycin of S. agalactiae strains isolated in our hospital from 1992 to 2001, (ii) to evaluate the in vitro activity of telithromycin against erythromycin-resistant S. agalactiae isolates, and (iii) to determine macrolide resistance mechanisms in erythromycin-resistant isolates.

The 1,462 isolates included in the study were recovered from 1992 to 2001 in the Clinical Microbiology Department of the Hospital Clínico San Carlos, Madrid, Spain. Only one isolate per patient was studied to avoid duplication. Organisms were identified by standard methods, including agglutination with latex (Slidex Strepto B; bioMérieux). The clinical sources were as follows: skin and soft tissues (631 isolates), urine (433 isolates), genital tract (221 isolates), respiratory tract (81 isolates), blood (66 isolates), and others (30 isolates). Routine antimicrobial susceptibility testing of the 1,462 isolates was performed by a broth microdilution procedure (Sensititre; Trek Diagnostic Systems, East Grinstead, England) according to the recommendations of the National Committee for Clinical Laboratory Standards (15). PCR was used to detect erm and mef genes in 102 erythromycin-resistant isolates (18). Erythromycin resistance phenotypes were determined by the double-disk diffusion method (17). The in vitro activity of telithromycin was compared with that of erythromycin, azithromycin, miocamycin, clindamycin, quinupristin-dalfopristin, and tetracycline against 156 erythromycin-resistant S. agalactiae strains by the National Committee for Clinical Laboratory Standards agar dilution method (15); 54 isolates were included from a previous study (5).

As shown in Fig. 1, the rate of resistance to erythromycin (14%) in 1992 declined significantly (P < 0.05) to 4.2% in 1993 and 2.5% in 1994. The frequency of this resistance remained low during 1995 and 1996 but increased significantly (P < 0.05) to 14.5% in 1998 and to 18% in 1999. An increasing trend in the rates of resistance to clindamycin was also observed, as the majority of our erythromycin-resistant isolates showed the constitutive macrolide-lincosamide-streptogramin B resistance (cMLS_B) phenotype. Erythromycin resistance rates ranging from 7.4 to 46% have recently been reported by several investigators (1, 7, 8, 12, 14, 16), and the frequency of erythromycin resistance seen among our isolates during the period 1999 to 2001 is similar to that found recently in Canada (6) and in the United States (13).

FIG. 1.

Frequency of resistance of S. agalactiae to erythromycin and clindamycin from 1992 to 2001.

The results of determination of phenotypes and genotypes of the 102 erythromycin-resistant isolates of this study are presented together with those of another 54 isolates previously described by our group (5) (Table 1). Erythromycin resistance was found to be due predominantly to the presence of an Erm(B) methylase. The inducible MLS_B (iMLS_B) and M phenotypes were detected in 28.2 and 3.8% of the strains, respectively. Most prevalent was the erm(B) gene, which was found in 81 cMLS_B phenotype isolates and in 29 iMLS_B isolates. The erm(A) gene was found in 43 cMLS_B isolates, in 29 iMLS_B isolates, and in 3 isolates bearing the M phenotype. mef(A) was found in six isolates with an M phenotype and in four isolates with an iMLS_B phenotype. The low prevalence (6.4%) of the mef(A) gene among our isolates agrees with that described by de Azavedo et al. (6). Erythromycin resistance in two strains was not associated with either the mef or the erm gene. As can be seen in Table 1, the following combinations of resistance genes were found: 35 strains carried both erm(A) and erm(B) genes, 3 carried both mef(A) and erm(B) genes, and 4 strains harbored both erm(A) and mef(A) genes.

TABLE 1.

Distribution of erythromycin resistance genes among 156 erythromycin-resistant S. agalactiae isolates with different phenotypes

Erythromycin resistance gene(s)	No. of strains with phenotypea having gene
	C	I	M
erm(B)	61	11	0
erm(A)	23	14	0
erm(B) + erm(A)	20	15	0
mef(A)	0	0	3
erm(B) + mef(A)	0	3	0
erm(A) + mef(A)	0	1	3
Unknown	2	0	0

^aC, constitutive; I, inducible; M, efflux.

Tetracycline resistance was common (85.2%) among the isolates tested. All isolates were also resistant to azithromycin. As previously reported by de Azavedo et al. (6), all of the cMLS_B phenotype isolates with erm(A) were highly resistant to the macrolides tested and to clindamycin. We did not find isolates susceptible to erythromycin and resistant to clindamycin, as reported elsewhere (6, 11, 20). Nevertheless, in line with a previous study (5), we recovered isolates (n = 20) for which the MICs of clindamycin were higher than those of erythromycin. Miocamycin and clindamycin showed good activity against isolates with the M phenotype, and quinupristin-dalfopristin inhibited all but one isolate at 0.5 μg/ml. Telithromycin was very active against all strains, irrespective of their mechanisms of macrolide resistance. The MICs of telithromycin at which 50 and 90% of the isolates tested were inhibited were the lowest of all the agents tested and inhibited 98% of strains at 0.25 μg/ml. MICs of telithromycin for the cMLS_B phenotype isolates carrying erm(B) were low (0.008 to 2 μg/ml), in contrast to the higher MICs (4 to 64 μg/ml) described by Jalava et al. (10) for the erythromycin-resistant Streptococcus pyogenes cMLS_B phenotype isolates with erm(B). To our knowledge, there are only two other studies (2; B. M. Wiley, L. Trpeski, S. Pong-Porter, J. de Azavedo, K. Weiss, R. Davidson, A. McGeer, D. E. Low, and the Canadian Bacterial Surveillance Network, Abstr. 40th Intersci. Conf. Antimicrob. Agents Chemother., abstr. 2161, 2000) of the activity of telithromycin against erythromycin-resistant S. agalactiae, although the numbers of isolates studied were lower. Our results are similar to those reported in both studies.

In order to prevent S. agalactiae infection in neonates, intrapartum antibiotic prophylaxis with penicillin or ampicillin is recommended. Clindamycin and erythromycin are the alternative agents for patients with a history of penicillin allergy (4). The change in antibiotic resistance increases the risk of neonatal S. agalactiae infection in penicillin-allergic mothers and stresses the need for continued surveillance of susceptibility of these organisms. Routine testing for susceptibility to macrolides and lincosamides should be performed because determination of erythromycin resistance phenotypes can be helpful in the selection of an appropriate alternative therapy for penicillin-allergic patients. The low prevalence of the M phenotype among erythromycin-resistant S. agalactiae isolates in our area raises the concern that neither clindamycin nor 16-membered macrolides are adequate alternatives. The excellent activity of telithromycin against macrolide-resistant S. agalactiae isolates suggests that it could be considered as an alternative to penicillin for prophylaxis and treatment of S. agalactiae infections.