ISOLATION, IDENTIFICATION AND BIOCONTROL OF SALMONELLA TYPHIMURIUM IN KARIESH CHEESE BY BACTEIOPHAGE

The study aims to assess the possibility of biological control on one of the most serious pathogenic microbes that found to infect Kariesh cheese, namely Salmonella typhimurium.  To achieve this object, firstly a total of 20 Kariesh cheese samples were collected randomly from various markets located at Cairo and exposed to microbiological isolation and identification of S. typhimurium. The obtained results revealed that, S. typhimurium was detected in 30% of surveyed market Kariesh cheese according to the strain identified by polymerase chain reaction (PCR) technique. Secondary, five sewage water samples were obtained from Fac. of Agric., Ain Shams Univ., and Shoubra EL-Kheima station of drinking and sewage water for specific bacteriophage isolation and morphology particles of Salmonella bacteriophage was examined by transmission electron microscope. Third, pasteurized skimmed buffalo’s milk was converted into experimental Kariesh cheese at 40oC by milk inoculation with 2% of freshly activated yoghurt bacterial starter culture and then milk was divided into 5 equal portions. The 1st portion considered as control. The 2nd, 3rd, 4th and 5th portions were contaminated with equal level (1%) of S. typhimurium suspension containing 105 colony forming units (CFU)/mL, previously isolated from foregoing surveyed Kariesh cheese samples, followed by adding phage suspension, 
from which isolated from sewage water, containing 108 plaque forming units (PFU)/mL at the levels of nil, 1, 2 and 3% respectively. All portions were separately incubated at the same temperature up to curdling. The curds were cut and individually filled into stainless steel moulds lined with cheese cloth and consolidated by a slight pressure for 24 h. The blocks of curd were then cut, dry salted using 2% NaCl (w/w) and packaged into plastic containers.  Experimentally, there were proportional reductions in lactic acid bacteria (LAB) population as the level of phage spiked into cheese milk increased, as which the reduction rate of LAB count during cold storage period (CSP) prolonging was however declined.    In terms of health safety, although the number of pathogen microbe added was gradually reduced due to the acid developed by prolonging the Cold Storage Period in the absence of phage, but it stilled present until the end of experimental period. While, the pathogen was completely eliminated within 7 days of cheese age when the phage suspension (108 PFU/mL) has been spiked at the level of 1% at least.  The contamination of experimental Kariesh cheese with S. typhimurium led to weaken the ability of cheese curd to drain whey as explained from the dry matter (DM) content which decreased due to the presence of pathogen and increased by the pathogen elimination with bacteriophage, which resulted also to increase the protein /DM content. The ash content reduced by both reasons, namely the contamination with S. typhimurium and/or the spiking level of phage suspension. The presence of S. typhimurium slowed the LAB population and acid production by them. Finally, as a conclusion, the spiking of Kariesh cheese milk with 1% Salmonella typhimurium phage suspension (108 PFU/mL) is quite enough to eliminate this microorganism when it present at the level of 1% suspension containing  105 CFU /mL.