Abstract
This research investigated the antimicrobial activity and antibiotic susceptibilities of nine Lactobacillus delbrueckii subsp. bulgaricus strains and nine Streptococcus thermophilus strains isolated from commercial yoghurt cultures. The antimicrobial activities of overnight culture strains against Bacillus cereus CECT 131, Bacillus subtilis ATCC 6633, Campylobacter jejuni NCTC 11351, Candida albicans ATCC 14053, Enterobacter aerogenes ATCC 13048, Escherichia coli ATCC 25922, Listeria monocytogenes ATCC 7644, Pseudomonas aeruginosa ATCC 9027, Salmonella typhimurium ATCC 14028 and Staphylococcus aureus ATCC 12600 were investigated using the disc diffusion method. The antibiotic resistance/susceptibility profiles of the strains were determined using antibiotic discs, which included ampicillin (10µg), ampicillin (25µg), bacitracin (10µg), clindamycin (2µg), clindamycin (10µg), erythromycin (10µg), erythromycin (15µg), gentamicin (10µg), gentamicin (120µg), nalidixic acid (30µ), neomycin (10µg), novobiocin (5µg), oxacillin (1µg), penicillin (10units), streptomycin (25µg), streptomycin (300µg), tetracycline (30µg) and vancomycin (30µg). The results of the study showed that while the Streptococcus thermophilus strain SY8, Lactobacillus bulgaricus strains LY6, LY8, LY9 and LY10 showed antimicrobial activity for all test microorganisms, Streptococcus thermophilus SY5 strains showed the weakest antimicrobial effect. All the Lactobacillus bulgaricus and Streptococcus thermophilus strains were resistant to oxacillin (1μg) and nalidixic acid (30μg). Indeed, the highest antibiotic susceptibility was seen with antibiotics such as ampicillin (25μg), clindamycin (10μg) and erythromycin (15μg).
Highlights
Lactic acid bacteria (LAB) are industrially important microorganisms and are used in various forms in industrial food fermentations
Overnight cultures of the Lactobacillus bulgaricus and Streptococcus thermophilus strains were prepared and antimicrobial activities against test microorganisms which are consist of Bacillus cereus CECT 131, Campylobacter jejuni NCTC 11351, Candida albicans ATCC 14053, Bacillus subtilis ATCC 6633, Enterobacter aerogenes ATCC 13048, Escherichia coli ATCC 25922, Listeria monocytogenes ATCC 7644, Salmonella typhimurium ATCC 14028, Pseudomonas aeruginosa ATCC 9027 and Staphylococcus aureus ATCC 12600 were evaluated by the disc diffusion method using 9 mm diameter antimicrobial activity discs (Schleicher & Schuell, Dassel, Germany)
Nine Lactobacillus bulgaricus and nine Streptococcus thermophilus strains isolated from commercial yoghurt starter cultures were tested
Summary
Lactic acid bacteria (LAB) are industrially important microorganisms and are used in various forms in industrial food fermentations. Lactic acid bacteria used in conventional fermented foods are gram-positive, facultative anaerobes, catalase negative, immobilized, lacking cytochromes and are non-spore forming (Carr et al, 2002; Mathur & Singh, 2005). LAB are reported to cover 17 genera: Aerococcus, Alloiococcus, Dolosigranulum, Enterococcus, Globicatella, Carnobacterium, Lactobacillus, Lactococcus, Leuconostoc, Melissococcus, Lactosphaera, Oenococcus, Pediococcus, Tetragenococcus, Vagococcus, Streptococcus and Weisella (Crowley et al, 2013). These bacteria, which show heterotrophic feeding patterns, can be found in coccus (spherical), rod (elongated) and oval shapes and tetrad formations and oval shapes and tetrad formation. LAB found in milk and milk products catabolize glucose in two ways: homofermentative and heterofermentative (Kandler, 1983)
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