Inhibition of Enteropathogens by Lactobacilli Strains Used in Fermented Milk

Abstract

Antibiotic resistance among enterococci and fecal streptococci was examined by testing 149 isolates from pork, water, and clinical material, as well as 50 strains of 13 known species, for resistance to 27 different antimicrobial agents. Tests were performed by using the MicroScan Pos MIC type 6 panels. Pork isolates exhibited less resistance than either water or clinical isolates to most antibiotics, although a larger proportion of pork isolates than others was resistant to tetracycline. Comparisons of antimicrobial-resistance patterns between enterococcal species revealed that Enterococcus faecium was most resistant to fi-lactam antimicrobials, especially ampicillin, whereas Enterococcus faecalis seemed to be the most resistant to the synergistic effects of antimicrobial combinations. Vancomycin resistance was observed in one Enterococcus hirae isolate from water. Enterococcal isolates from any of the sources tested did not show multiple resistance to antibiotics (such as gentamicin, ampicillin, streptomycin, and vancomycin) used to treat serious infections caused by gram-positive cocci. Recent attention has focused on enterococci because of their remarkable and increasing resistance to antimicrobial agents (13). This resistance allows them to survive in environments in which antimicrobial agents are heavily used. Antimicrobials are given to food animals, such as swine, to improve their growth rate and feed conversion. However, many people believe that the use of antimicrobials in humans or animals is often followed by appearance of resistant microorganisms (14). Studies by Cohen and Tauxe (2) suggested that the antimicrobial drugs to which food animals were exposed provided selective pressure that led to the appearance and persistence of drug-resistant strains. Specifically , these authors associated the occurrence of certain drug-resistant Salmonella sp. to antimicrobial use in food animals. If antimicrobial use in food animals is linked to antimicrobial resistance in Salmonella, Enterococcus species in the intestinal tract might also be expected to develop similarly elevated resistance patterns. Antimicrobial resistance in enterococci can be divided into two general types, intrinsic and acquired. Intrinsic resistance is present in all or most strains of those species, and the genes appear to reside on the chromosome. Intrinsic resistance includes resistance to semisynthetic, penicillinase-resistant penicillins, cephalosporins, and low levels of aminoglycosides and clindamycin. Acquired resistance results from a mutation in cellular DNA or acquisition of new DNA. Examples of acquired resistance include resistance to chlorampenicol, erythromycin, tetracycline, high levels of aminoglycosides and clindamycin, penicillin by means of penicillinase, fluoroquinolones, and vancomycin (13). Several researchers have shown that differences in antimicrobial susceptibility exist between Enterococcus faecium and En-terococcus faecalis (4,10,11). The possible existence of similar susceptibility differences among the more recently described species of Enterococcus needs to be determined. An assessment of possible correlations between a certain species and a given susceptibility pattern could provide valuable information (75). Few data are available on antimicrobial-resistance patterns of isolates from widely divergent environmental sources studied in a single laboratory by using a common procedure. In this study, enterococci and fecal streptococci were collected from three sources: pork, including pork carcasses and processed pork products; water, including samples from rivers, lakes, and wells; and clinical material from several sources. These samples, as well as known strains, were tested for resistance to 24 antimicrobials and 3 synergy screens with MicroScan Pos MIC type 6 panels. The results were examined for differences in resistance patterns between entero-cocci from the three different sources (pork, water, clinical). Differences in resistance patterns between different species of enterococci and fecal streptococci also were examined. MATERIALS AND METHODS Cultures Fifty named strains of 13 species of enterococci and Strepto-coccus bovis and Streptococcus equinus were collected from various sources, and their identities were confirmed (7). Cultures were also isolated from pork carcasses during slaughter and from fresh and spoiled pork products. All pork sample cultures were isolated according to Knudtson and Hartman (8). Enterococci from water samples (6) were isolated by the membrane filter method (7). Clinical isolates were collected from patients at one hospital over a 2-year period (6). Primary isolation on blood agar plates revealed colonies that comprised gram-positive cocci. Catalase, bile esculin, and Lancefield grouping tests confirmed that the isolates were group D enterococci. Species identifications were carried out as indicated by Knudtson and Hartman (7).