Phenotype and genotype of some clinical group B streptococcus isolates resistant to erythromycin in Egypt

ABSTRACT

The aim of the study was to determine susceptibility of group B streptococci [GBS] to commonly used antibiotics and to determine the phenotype of macrolide resistance correlating it with the genotype. 220 group B streptococcus strains were subjected to antibiotic sensitivity disc diffusion method to penicillin, ampicillin, ceftriaxone, tetracyclines, erythromycin [E] and clindamycin [CD]. Erythromycin resistance phenotype was done by double disc diffusion test. Erythromycin resistance mefA, ermB, and ermTR genes were determined using multiplex PCR. The results revealed that all strains were sensitive to penicillin, ceftriaxone and ampcillin. 161 [73%], 38 [17%] and 24 [11%] strains were resistant to tetracyclines, erythromycin and clindamycin respectively. 15 [39.5%] were resistant to E alone [M phenotype]. M phenotype strains were associated with mefA gene. 23 [60.5%] isolates were resistant to both E and clindamycin [CD]. 9 [23.7%] strains were inducibly resistant to clindamycin [iMLSB] and all had ermTR gene.. 14 [36.8%] were constitutively resistant to clindamycin [cMLSB]. cMLSB strains have different genotypes. 7 strains were ermB genotype. 4 strains were ermTR genotype. 2 strains had both ermB and ermTR genes while only one strain had both ermB and mefA genes. Only 1 strain was resistant to clindamycin but sensitive to E. MefA was detected in 16 [42%] E resistant strains. ErmTR was also detected in 16 [42%] E resistant strains. ErmB was detected in 10 [26%] E resistant strains. 31[82%] E resistant strains contain single resistance gene while 7[18%] resistant strains contain more than one resistance gene. The study findings conclude that GBS isolates remain uniformly susceptible to penicillin and ampicillin but he erythromycin resistance has reached substantial level. The study recommends that testing of susceptibility to erythromycin and clindamycin by double disc diffusion method should be performed in individual cases when considered as alternatives for prophylaxis and treatment of GBS infection or colonization because strains with inducible MLSB cannot be detected with the conventional disc diffusion method: