Abstract
The laboratory studies of the cultural isolation revealed that two isolates of Lactobacillus acidophilus were isolated from five soft cheese samples that were manufactured locally according to the farmers production procedure .Colonies of E.coli O157:H7 were isolated from raw milk samples by a conventional direct plating on selective enrichment CT-SMAC agar and tested serologically for the presence of O157 and H7 antigens using the commercial available latex agglutination kits .The results revealed that E.coli O157:H7 was resist to the crude bacteriocin that was extracted from L .acidophilus isolate NO.1 due to the nature of their cell wall, while when the outer membrane of E.coli O157:H7 was injured by cooling at 4°C for 6 hours , the permeation of bacteriocin to the cytoplasmic membrane was facilitated and caused inactivation and death of such pathogenic organism .The results revealed that no growth of E.coli O157:H7 with no visible turbidity in the nutrient broth with bacteriocin that diluted to 1\2 ,1\4 ,1\8 were observed .Bacteriocin that diluted to 1\8 had been recognized as the minimum inhibitory concentration of the bacteriocin.Both the dilution factor of bacteriocin and time exposure to bacteriocin at refrigeration storage temperature of milk had a significant (P˂0.05) influence on its antimicrobial potency against the viability of stressed E.coli O157:H7 from minutes 30 to 120 .The viability of E.coli O157:H7 subjected to bacteriocin that diluted 10 times(1:10) in both raw and sterile milk was efficiently eliminated after 90 minutes and 60 minutes of exposure at refrigeration temperature respectively.
Highlights
Bacteriocins from Lactobacillus acidophilus are known to be bactericidal to closely related gram positive bacteria and few reports are available on their activity against gram negative microorganisms (7) Bacteriocins bactericidal mode of action usually acting on the bacterial cytoplasmic membrane .(8) A large numbers of studies were published in recent decades on bacteriocins against gram positive bacteria but few researches concerning lactic acid bacteria (LAB) bacteriocins against gram negative bacteria and this is the main reason for this study to highlight its effect against E.coli O157:H7
The average diameter of the inhibition zone of crude bacteriocin that was produced by the Lactobacillus acidophilus isolate NO.1 against the sensitive strain Lactobacillus acidophilus R0052 was 12 mm and showed a potency of 100% while the inhibition zone against the test organism E.coli O157:H7 was 0.0 mm which means was resistant to the crude bacteriocin (Table 2) 0% potency
The results revealed that no growth of E.coli O157:H7 with no visible turbidity in the broth was observed with bacteriocin that diluted to 1/2, 1/4 and 1/8
Summary
E.coli O157:H7 was first recognized as a human pathogen in 1982 when two outbreaks of haemorrhagic colitis (HC) in Michigan and Oregon (USA) were associated with the consumption of under cooked ground beef that had been contaminated with such organism (1).Since ,this pathogen has caused many food borne outbreaks of Haemorragiccolitis (HC) and Haemolytic Ureamic Syndrome (HUS) worldwide (2) .Human beings and cattle carry the pathogen in their intestines and their faeces are considered as a source of contamination of foods ,water and environment .The faeces may contaminate udders and milking equipment’s and get into milk during milking and handling under inadequate hygiene practices(2) .Most of food borne infections associated with this pathogen included consumption of raw milk (3).Various modifications of agar diffusion assays as well as test procedures in liquid media were widely used and partly standardized for testing the activity of antimicrobial agents (4).The use of lactic acid bacteria (LAB) especially the isolates from dairy products such as L . acidophilus and \or their antagonistic metabolism such as bacteriocin is an example of biopreservation (5). Bacteriocins from Lactobacillus acidophilus are known to be bactericidal to closely related gram positive bacteria and few reports are available on their activity against gram negative microorganisms (7) Bacteriocins bactericidal mode of action usually acting on the bacterial cytoplasmic membrane .(8) A large numbers of studies were published in recent decades on bacteriocins against gram positive bacteria but few researches concerning LAB bacteriocins against gram negative bacteria and this is the main reason for this study to highlight its effect against E.coli O157:H7. The main objectives of this study were to isolate L .acidophillus from locally homemade soft cheese and to evaluate the antimicrobial potency of crude bacteriocin and its minimum inhibitory concentration against the survival of both unstressed and cold stressed E.coli O157:H7, beside that studying its effectiveness in both raw and sterile milk on the survival of E.coli O157:H7 at refrigeration storage
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