Screening Fecal Enterococci from Greek Healthy Infants for Susceptibility to Antimicrobial Agents Ekaterini I. Kirtzalidou, 1 Evdokia K. Mitsou, 1 Paraskevi Pramateftaki, 2 and Adamantini Kyriacou 1 Enterococci are among the first lactic acid bacteria to colonize the neonatal gastrointestinal tract, but they are also characterized as significant nosocomial pathogens. The aim of this study was to investigate the incidence of antibiotic resistance in enterococci isolated from neonates’ gut microbiota as well as the presence of genetic determinants encoding for certain antibiotic resistance traits. A total of 263 fecal samples derived from 97 infants were collected on day 4, 30, and 90 after delivery. Enterococcus faecalis was the most frequently identified species (54.6%) followed by E. faecium, while E. casseliflavus/E. flavescens and E. gallinarum were also traced. The isolates were examined for their resistance to 12 antibiotics. Rifampicin resistance was the highest observed (53.2%), followed by resistance to tetracycline (42.0%), erythromycin (35.7%), and vancomycin (11.2%). Multiresistant strains were highly prevalent. Only intrinsic vancomycin resistance (vanC1 and vanC2/C3) was traced. The ermB gene was detected in 49 out of 96 erythromycin-resistant isolates, while tet genes were detected in 51 out of 113 tetracycline-resistant strains, with tet(L) being the most frequently observed. In conclusion, antibiotic-resistant enterococci are already established in the fecal microbiota of healthy neonates, from the first days of an infant’s life. Introduction E nterococci are gram-positive, facultative anaerobic bacteria, members of the commensal microbiota of the intestinal tract of both animals and humans. In fact, they are among the first lactic acid bacteria to colonize the neonatal gastrointestinal tract. 15 On the other hand, enterococci are also opportunistic pathogens; despite their low virulence, studies have documented that they are very significant nosocomial pathogens. 23 The intestinal microbiota is a po- tential source of pathogens, causing nosocomial infections, including bacteraemia, endocarditis, urinary tract infec- tions, and contamination associated with surgery. The oc- currence of large number of resistant bacteria within this flora is highly undesirable due to the risk of obstructing the effect of antibiotic treatment. 27 Enterococci express intrinsic resistance against cephalo- sporins and semisynthetic penicillins and acquired resistance against aminopenicillins and glycopeptides, 5,30 probably due to the use of broad-spectrum antibiotics (cephalosporins) or multiantibiotic regimens. 22 Enterococci acquire resistance to antimicrobial agents through the transfer of plasmids and transposons, chromo- somal exchange, or mutation. 29 The resistance to vancomy- cin, tetracycline, and erythromycin is encoded by genetic determinants usually harbored in transferable genetic mate- rial. 40 To date, eight types of vancomycin resistance have been described in enterococci (vanA, vanB, vanC1, vanC2/C3, vanD, vanE, vanG, and vanL), each of them associated with a different ligase gene, resulting in a decrease or inhibition of the glycopeptide action. vanC-type resistance is an in- trinsic, nontransferable property of Enterococcus gallinarum and E. casseliflavus/flavescens, related to the chromosomal presence of the species-specific genes vanC1 and vanC2/C3, respectively. All other types of vancomycin resistance are acquired and transferable characteristics, with vanA being the most clinically important. 25,43 Erythromycin resistance in enterococci is mainly associated with the presence of ermB gene on the conjugative transposon Tn917, which inhibits erythromycin binding by methylation of 23S rRNA. 21,25 Re- sistance to tetracycline in Enterococcus spp. can be mediated by energy-dependent efflux of tetracycline [tet(K) and tet(L) genes], by ribosomal protection from the action of tetracy- cline [tet(M), tet(O) and tet(S) genes] and by unknown mechanisms [tet(U) gene]. 7 In certain bacteria, tet(M) and tet(S) are harbored in conjugative transposons, implying a high potential for genetic exchange, especially in environ- ments with high bacterial concentrations such as the human 1 Department of Nutrition and Dietetics, Harokopio University, Kallithea, Greece. 2 Wine Institute of Athens, National Agricultural Research Foundation (N.AG.RE.F), Lykovrissi, Greece. MICROBIAL DRUG RESISTANCE Volume 18, Number 6, 2012 ª Mary Ann Liebert, Inc. DOI: 10.1089/mdr.2012.0028 578