The in vitro effects of lactic acid bacteria screened from gastrointestinal tracts of Lates niloticus on E. coli and Salmonella spp.

Abstract

Biopreservation systems in foods are of increasing interest in the food industry. It is the use of naturally occurring microorganisms and/or their inherent antibacterial compounds of defined quality and quantity to extend shelf life and to enhance the safety of foods. Bacteriocinogenic LAB and/or their isolated bacteriocins are considered safe additives, useful to control the frequent development of pathogens and spoilage microorganisms in foods. The spreading of bacterial antibiotic resistance and the demand for products with fewer chemicals create the necessity of exploring new alternatives, in order to reduce the use of therapeutic antibiotics. Bacteriocins are indicated to prevent the growth of undesirable bacteria in a food-grade and more natural way, which is convenient for health and accepted by the community. This study was carried out to screen lactic acid bacteria from gastrointestinal tracts of L. niloticus fish and to assess their in vitro effects on S. enteritica and E. coli. S. enteritica was chosen considering its being the causal agent of the largest number of enteric infections in the world, while E. coli as an indicator microorganism. A randomized complete block design was used. Random sampling was carried out at the landing sites on L. Victoria. A 25g portion of each GIT sample was aseptically obtained from fish followed by blending in 225ml sterile peptone water for two minutes in a blender. For bacterial enumeration and isolation, threefold serial dilutions of gut homogenates was pour plated in and spread plated on the surface of dry RMS agar and incubated at 37°C for 48 hours. The cultures of LAB were purified and identified (phenotypically and biochemically). The average weight of small fish sampled during the wet season was 667.9g that gave an average colony forming units/g of 9.2x 103; medium averaging 1485.1g had an average of 2.1x104cfu and big averaging 3210.8g had 4.9 x 104cfu. In the dry spells, the small fish averaging 614.6g had 6.7 x 103, medium averaging 1392.9g had an average of 1.7x104cfu and big averaging 2756.3g had 3.4 x 104cfu. A total of 96 fish samples were analyzed and 9 Lactobacillus isolates were identified, out of these four (B4, M4, G4 and E4) were identified to have anti-bacterial properties. The Lactobacillus isolates showed an inhibitory spectrum against the indicator organisms tested. Lactobacillus isolate B4 exhibited the highest degree of inhibition against S. enteritica and E .coli followed by M4, G4 and E4 respectively. In conclusion, this study showed availability of lactic acid bacteria in the gut of L. Victoria Nile perch and can be used as a good source of potential bio preservative. Further research required to identify these LABs to species level and the metabolites responsible for inhibition.