Object: Lactobacillus acidophilus is a nonpathogenic member of gastrointestinal tract and it is widely used in fermented dairy products. This study aimed to assess the antimicrobial potential of two strains of L. acidophilus on some pathogenic bacteria frequently causing diarrhea or gastroenteritis. Methods: The antibacterial activity cell free supernatant (CFS) of two control standard strains of L. acidophilus (L. acidophilus-la5 and L. acidophilus against five control standard strains of bacteria causing diarrhea; Enterotoxogenic Escherichia coli (ETEC), Enterohaemorrhagic E. coli O 157 :H 7 (EHEC) Salmonella typhimurium Shigella flexeneri and Staphylococcus aureus were determined using agar well diffusion method. The sensitivity of the pathogenic bacteria to the CFS of each L. acidophilus in relation to time was determined by standard plate count .The antibiotic susceptibility tests of 20 antibiotics against tested organisms with and without CFS were assessed by disc diffusion method. The minimum inhibitory concentration (MIC) of ciprofloxacin with and without CFS was determined by tube dilution method. Results: Both Lactobacilli strains decrease the colony count of tested strains by more than 90% after 60 min contact time. Both Lactobacilli strains significantly improve the antibacterial effect of tested antibiotics against ETEC, S. typhimurium and S. aureus, and S. flexeneri (P< 0.05). The MIC of ciprofloxacin alone against all tested strains was 15.625 μg/ml, while when combined with both Lactobacilli CFSs, the MIC decreased significantly to 0.488 μg/ml for ETEC, S. typhimurium, and S. flexeneri and to 0.977 μg/ml for EHEC and S. aureus (P= 0.000).

Streptococcus mutans is a Gram-positive facultative anaerobic bacterium that belongs to the mutans streptococci group and is considered to be an important pathogen associated with human dental caries. In this study, 106 S. mutans isolates were isolated from 140 clinical samples. The 106 isolates comprised 56 isolates from soft caries lesion, 50 isolates from dental plaque samples, the latter included 30 isolates from plaque of caries active and 20 isolates from plaque of caries-free cases. The isolates showed sticky and rough colonies on TYCSB media. Isolates produced alpha hemolysis on blood agar. Isolates were catalase-negative and able to ferment mannitol, sorbitol, inulin and sucrose. The identification of S. mutans was confirmed by the detection of the S. mutans-specific glucosyltransferase gene (gtfB) using PCR specific primers for S. mutans gtfB gene. Isolates were tested for their sensitivity to diverse antimicrobial agents. It was found that 99% of the isolates were sensitive to ofloxacin and levofloxacin and 88.5% of isolates was sensitive to chloramphenicol, however, 80.2%, 75.5% and 65% of the isolates were resistant to ceftriaxone, cefotaxime and erythromycin, respectively. It was recorded that 90.5% and 85.8% of the isolates were sensitive to linezolid and tetracycline, respectively, and were highly resistant to clindamycin with a frequency of 95.3%. Based on the statistical analysis using the chi-square test, there was a significant difference in antimicrobial susceptibility profiles among the three groups with higher frequencies among soft caries lesion isolates (P < 0.0001). Therefore, the extensive use of antimicrobials in treatment and prophylaxis in dental caries must be controlled.

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