Milk and dairy products are the most consumed and important resources in the daily diet of human. They also provide a considerable part of the public’s needs for nutrients because of having a variety of vitamins and minerals including calcium.
The attitude of consumers about the side effects of additives in the dairy industry increases their tendency toward the consumption of milk, traditional dairy products and products made from unprocessed or semi-processed milk. Because of the possible presence of human pathogenic microorganisms, consumption of products produced from raw milk can be harmful (Verraes et al. 2015).
In this study, from a total 150 samples of dairy products, 16 samples (10.66%) were contaminated to B. cereus. These results are somewhat in accordance with the findings of Reyes et al. (2013) who reported that from 260 samples of milk and dairy products, 24.23% were contaminated to this bacterium (Reis et al. 2013).
In this study, just one (3.33%) of 30 samples of tested ice cream was detected as contaminated with B. cereus. Messelhausser et al. (2010) were carried out a study on 809 ice cream samples and reported that 62.7% of samples were contaminated with B. cereus (16). However, among the performed studies about the contamination of ice cream with B. cereus in Iran, Pour-Mohammadi et al. (2002) and Shakerian et al. (2005) isolated this bacterium in 1.5% and 12% of samples, respectively.
So, the present study was not inconsistent with Messelhausser et al. but in accordance with the studies was performed in Iran. The difference between the results of present research with the findings of Messelhausser et al. may be related to different hygienic conditions in various stages of production, packaging and distribution in different countries.
From 30 traditional cheeses that were evaluated in this study, no positive results were found in the samples. However, according to the findings of Williams et al. (2010) in Scotland, 28% of cheese samples produced from raw milk was contaminated with B. cereus. Since B. cereus can’t tolerate the salt concentration more than 7.5%, this contradiction may be due to high salinity in Lighvan cheese (11%), which is known as a type of salty high-fat cheese.
In the present study, 6 (20%) of 30 curd samples were contaminated with B. cereus. Most of the production process of traditional curd is performed manually using traditional equipments. It also air-dried in sunlight. So, all of these processes can increase the risk of product contamination by spoilage microorganisms especially B. cereus which can easily contaminate the product by dust.
From 30 samples of Doogh evaluated in this study, 10 (30%) samples were contaminated with B. cereus. However, Keshtkar et al. (2012) evaluated the samples of pasteurized dairy products that were marketed in the cities of Tehran, Isfahan, Shiraz, Semnan and Shahr-e Kord and they found that 16.66% of doogh samples contaminated with B. cereus. The lack of inconsistency between findings can be explained by pasteurized dairy products evaluated in the study of Keshtkar et al. and unpasteurized samples with manual and traditional production in the present study. So, production processes and the use of unpasteurized milk can play a role in the contamination level of products.
In the present study, from 30 samples of traditional butter, there was no contamination with this bacterium. In a study by Faramarzi et al. (2011) about the contamination of traditional butter with pathogenic microbes, no positive sample was detected as contaminated with B. cereus. This may be due to the weight of B. cereus spores which can be deposited in the solution made from the butter.
The findings of this study about the presence of ces gene in the isolates showed that 14 (87.5%) of 16 samples contained the ces gene. However, Messelhausser et al. (2010) reported that 24 (4.7%) of 508 isolates from ice cream samples have the cereulide producing gene (ces). In another study by Samapundo et al. (2011) on cooked pasta, mince and lasagna in Belgium, none of 81 isolates of B. cereus expressed ces gene for production of emetic toxin.
Cui et al. (2016) reported that from 9.8% of B. cereus isolates in raw milk samples, 1.1% contained ces gene. Since there is no similar study about the presence of ces gene in curd and doogh, the possible reason for the high content of this gene in these products can also be associated with sanitary conditions in their production.
The study of antibiotic susceptibility pattern of isolates showed that most of them were resistant against penicillin, cefazolin and co-trimoxazole. But they were sensitive against gentamicin, ciprofloxacin and clindamycin and semi-sensitive against vancomycin and erythromycin. The findings of Jalalpour & Abusaeedi (2009) were somewhat similar to results of present study. They found that all strains were resistant against the selected antibiotic against Bacillus infections, which was penicillin. However, they have a significant sensitivity against vancomycin, tetracycline and erythromycin. It should be noted that the antibiotic selective stress in each area could be the reason for similarities or differences in antibiotic pattern. In addition, excessive use of antibiotics may lead to resistant strains in the environment.