Fifty-five strains of lactic acid bacteria (LAB) were isolated from 50 samples of raw milk. The isolates were classified as 32 Lactococcus (19 isolates were identified to species Lactococcus lactis subsp. lactis and 13 isolates as Lactococcus lactis subsp. cremoris); while 23 isolates were classified as Lactobacillus (15 isolates were identified to species Lactobacillus delbrueckii subsp. lactis and 8 isolates as Lactobacillus acidophilus). Ten isolates were selected ( 3 isolates of Lactococcus lactis subsp. lactis; 2 isolates of Lactococcus lactis subsp. cremoris; 3 isolates Lactobacillus delbrueckii subsp. lactis and 2 isolates Lactobacillus acidophilus) and employed to produce bacteriocins in different media (milk permeate supplemented MPS, MRS, M17 and TY broth media). The preferable medium for production of bacteriocin extracts was the MPS (conc. 650-750 AU/ml), followed by M17 (conc. 400-550 AU/ml), MRS (conc. 350-500 AU/ml) and TY (conc. 250-450 AU/ml) broth media. All bacteriocin extracts were active against indicator bacteria (G+: Listeria monocytogenes, Staphylococcus aureus & Bacillus subtilis and G־: Salmonella typhemorium, Escherichia coli & Pseudomonas fluorescens). Four extracts were selected from the previous 10 extracts of MPS medium (which gave the highest inhibition zone against indicator bacteria) to study some characteristics of bacteriocin extracts. They were resistant to heat treatments up to 100ºC for 30 min. and remained full active over a pH range 2 to 8 at 28ºC /16 hr., also they were completely destroyed by pepsin and trypsin, except the bacteriocin extract produced by Lactococcus lactis subsp. lactis. The activity of bacteriocin extracts was persisted for 90 days at -15ºC but they were stable at ~5ºC until 30 days and gradually decreased throughout 60 to 90 days from storage at ~5ºC. The activity of bacteriocin extracts was gradually decreased during storage at ~25ºC until 60 days and completely lost up to 90 days of storage. |
