1. Background
Foodborne diseases are one of the serious problems in developed and developing countries. Every year, more than 100 million people are afflicted by foodborne and waterborne diseases in the world, especially people with immune system deficiency and malnutrition (1, 2). Different species of Salmonella, Listeria and Yersinia are related to the diseases, which can be transmitted through consuming contaminated fish, dairy products, vegetables and meat (3, 4).
Epidemiological studies have shown that foods of animal origin are among the most important sources of foodborne diseases (5, 6). Food products are usually contaminated with pathogens during the production, processing, distributing and retailing in the market (7). Different studies showed that the frequencies of beef contaminations with human pathogens such as Salmonella were not the same (8-10).
Salmonellosis is the major cause of foodborne infections and the second-most common foodborne illness after Campylobacter infection (11). Salmonella infection in human is often resulted from ingestion of contaminated foods such as beef, pork, egg, milk, seafood, and fresh products (11-15). Pathogens can survive in the food products, especially in meat, until distributed in the markets (16, 17). One of the best methods to prevent food products contamination is packaging. In the industrialized countries, food products are mostly distributed in the form of packages. In Iran, some food products such as chicken and beef are traditionally distributed unpackaged. Meat products are one of the most consumed foods; hence, they have a key role in transmission of Salmonella to humans. There are annually thousands of reported cases of foodborne diseases related to Salmonella in Iran. Many of Salmonella specie isolated from meat products are multidrug resistant strains, which can cause serious problems (18, 19).
2. Objectives
The aim of this study was to compare the contamination rate of Salmonella in packaged and unpackaged chicken and beef products as well as the effect of packaging on contamination prevention.
3. Materials and Methods
3.1. Sample Collection
A total of 379 samples, including 189 beef and 190 chicken samples were collected from different stores in Tehran. All samples were kept at 4°C before and during transferring to the laboratory.
3.2. Isolation, Identification and Serotyping of Salmonella
Identification of Salmonella was performed according to ISO-6575 (20). From each sample, 25 g was placed in a sterile stomacher bag and 225 mL buffered peptone water, pH 7.0 (Merck, Germany) was added to it. Samples were then homogenized using a stomacher for at least 2 minutes, followed by incubation at 37°C for 24 hours. Afterwards, 0.1 mL of pre-enriched broth was transferred into 10 mL of Rappaport-Vassiliadis medium (Oxoid, UK) and incubated at 42°C for 24 hours.
The enriched samples were then plated onto Hektoen enteric agar (Sigma, USA) and incubated at 37°C for 24 hours. Colonies on Hektoen enteric agar were then identified using biochemical tests such as oxidase reaction, acid production from manitol, ONPG test, H2S and indole production, and urease and lysine decarboxylase activity. For final identification and verification, isolated strains were cultured on differential media such as Simmon’s citrate agar, urea broth, lysine iron agar, methyl red and sulfide indole motility (SIM), and then incubated at 37°C for 24 hours.
3.3. Statistical Analysis
Statistical analysis was performed using SPSS software version 19. The P values < 0.05 were considered statistically significant.
4. Results
Out of 379 samples, 134 (32.7%) were contaminated (Table 1) and 13 serotypes were identified (Table 2). The most isolated serotypes were S. thompson (67.7%), S. haardt (6.5%) and S. veyle (4.8%), respectively (Table 2). Of S. thompson serotype, 65 were isolated from beef and 19 from chicken. Rates of isolated Salmonella spp. from unpackaged and packaged beef and chicken samples were 22.3%, 16.4%, 46%, 43.3%, respectively (Table 3). Isolation rate of Salmonella spp. from unpackaged chicken was 46%, which was significantly higher than unpackaged beef with the rate of 22.3%. The statistical analysis difference rate among different sources of chicken and beef Salmonella spp. isolates was P < 0.05. Serotypes were more variable in unpackaged samples (Table 4). The contamination rates of samples were not related to the collection region.
The most isolated serotype from the packaged beef and chicken was S. thomson with the rate of 4.8% and 28.6%, respectively. S. enteritidis (16.7%), S. haardt (12.5%) and S. virginia (33.3%) were detected in the packaged chicken samples (Table 5) and S. anatum and S. meleagridis (100%) were detected only in the packaged samples, S. paratyphi C and S. kentucky (100%) were detected only in the unpackaged samples (Table 4).
The most isolated serotypes in the unpackaged beef and chicken samples were S. thampson, S. paratyphi, S. haard, S. enteritidis, and S. virigina was detected in the unpackaged chicken samples. S. voyle (60%), Salmonella group II (100%) and S. kentucky (100%) were isolated from the unpackaged beef samples. Salmonella group F, S. meleagrides were not isolated from the unpackaged samples (Table 5).
Table 1. Prevalence of Salmonella spp. in Beef and Chicken Samples a
| Type of Sample | Positives | Negative | Total |
|---|---|---|---|
| Chicken | 86 (45) | 104 (55) | 190 (100) |
| Beef | 38 (20.2) | 151 (79.8) | 189 (100) |
| Total | 124 (32.7) | 255 (67.3) | 379 (100) |
a Data are presented as No. (%).
Table 2. Distribution of Salmonella Serotypes in Beef and Chicken Samples a
| Serotype | Type of Samples | ||
|---|---|---|---|
| Beef | Chicken | Total | |
| S. thompson | 19 (50) | 65 (75.6) | 84 (67.7) |
| S. paratyphi C | 1 (2.6) | 2 (2.3) | 3 (2.4) |
| S. anatum | 1 (2.6) | 0 (0) | 1 (0.8) |
| S. veyle | 5 (13.2) | 0 (0) | 5 (4) |
| S. enteritidis | 1 (2.6) | 5 (5.8) | 6 (4.8) |
| S. haardt | 2 (5.3) | 6 (7) | 8 (6.5) |
| S. virginia | 0 (0) | 3 (3.5) | 3 (2.4) |
| Salmonella group II | 1 (2.6) | 0 (0) | 1 (0.8) |
| S. meleagridis | 2 (5.3) | 0 (0) | 2 (1.6) |
| S. typhimurium | 2 (5.3) | 1 (1.2) | 3 (2.4) |
| S. untypable | 2 (5.3) | 4 (4.7) | 6 (4.8) |
| S. kentucky | 1 (2.6) | 0 (0) | 1 (0.8) |
| Salmonella group F | 1 (2.6) | 0 (0) | 1 (0.8) |
| Total | 38 (100) | 86 (100) | 124 (100) |
a Data are presented as No. (%).
| Type of Sample | Positives | Negative | Total |
|---|---|---|---|
| Packaged beef | 11 (16.4) | 56 (83.6) | 67 (100) |
| Unpackaged beef | 27 (22.3) | 93 (77.7) | 122 (100) |
| Packaged chicken | 29 (43.3) | 38 (56.7) | 67 (100) |
| Unpackaged chicken | 57 (46) | 66 (45) | 123 (100) |
| Total | 124 (32.7) | 223 (58.8) | 379 (100) |
a Data are presented as No. (%).
b x2 = 6.91, d. f. = 3, P = 0.75.
Table 4. Distribution of Salmonella Serotypes in the Packaged and Unpackaged Samples a
| Serotype | Type of Sample | ||
|---|---|---|---|
| Packaged | unpackaged | Total | |
| S. thompson | 28 (70) | 56 (66.7) | 84 (67.7) |
| S. paratyphi C | 0 (0) | 3 (3.6) | 3 (2.4) |
| S. anatum | 1 (2.6) | 0 (0) | 1 (0.8) |
| S. veyle | 2 (5) | 3 (3.6) | 5 (4) |
| S. enteritidis | 1 (2.6) | 5 (5.8) | 6 (4.8) |
| S. haardt | 1 (2.6) | 7 (8.3) | 8 (6.5) |
| S. virginia | 1 (2.6) | 2 (2.4) | 3 (2.4) |
| Salmonella group II | 0 (0) | 1 (1.2) | 1 (0.8) |
| S. meleagridis | 2 (5) | 0 (0) | 2 (1.6) |
| S. typhimurium | 0 (0) | 3 (3.6) | 3 (2.4) |
| S. untypable | 3 (7.5) | 3 (3.6) | 6 (4.8) |
| S. kentucky | 0 (0) | 1 (1.2) | 1 (0.8) |
| Salmonella group F | 1 (2.6) | 0 (0) | 1 (0.8) |
| Total | 40 (100) | 84 (100) | 124 (100) |
a Data are presented as No. (%).
Table 5. Distribution of Salmonella Strains in the Packaged and Unpackaged Beef and Chicken Samples a
| Serotype | Chicken | Beef | ||
|---|---|---|---|---|
| Packaged | Unpackaged | Packaged | Unpackaged | |
| S. thompson (n = 84) | 24 (28.6) | 41 (48.8) | 4 (4.8) | 15 (17.9) |
| S. paratyphi C (n = 3) | 0 (0) | 2 (66.7) | 0 (0) | 1 (33.3) |
| S. anatum (n = 1) | 0 (0) | 0 (0) | 0 (0) | 1 (100) |
| S. veyle (n = 5) | 0 (0) | 0 (0) | 2 (40) | 3 (60) |
| S. enteritidis (n = 6) | 1 (16.7) | 4 (66.7) | 0 (0) | 1 (16.7) |
| S. haardt (n = 8) | 1(12.5) | 5 (62.5) | 0 (0) | 2 (25) |
| S. virginia (n = 3) | 1 (33.3) | 2 (66.7) | 0 (0) | 0 (0) |
| Salmonella group II (n = 1) | 0 (0) | 0 (0) | 0 (0) | 1 (100) |
| S. meleagridis (n = 2) | 0 (0) | 0 (0) | 2 (100) | 0 (0) |
| S. typhimurium (n = 3) | 0 (0) | 1 (33.3) | 0 (0) | 2 (66.7) |
| S. untypable (n = 6) | 2 (33.3) | 2 (33.3) | 1 (16.7) | 1 (16.7) |
| S. kentucky (n = 1) | 0 (0) | 0 (0) | 0 (0) | 1 (100) |
| Salmonella group F (n = 1) | 0 (0) | 0 (0) | 1 (100) | 0 (0) |
a Data are presented as No. (%).
5. Discussion
Salmonellosis is one of the most important foodborne diseases (23). High prevalence of Salmonella species in chicken and beef samples obtained in this study was similar to the previous studies (11, 24-26). Salmonella spp. infections are usually caused by handling or consuming contaminated food products (27-30). In this study, the prevalence of Salmonella spp. in chicken meat samples was higher than beef samples. After death, the level of pH is reduced in animal tissues, leading to the decrease of bacterial growth in beef samples. A few previous studies have shown that bacteria normally grow more slowly in meat products with low pH levels (31).
Serotypes such as S. thampson, S. typhimurium, S. enteridis, S. infantis, S. paratyphi B were isolated from chicken and beef samples in the previous studies (32). The most isolated serotypes in this study were S. thompson, S. paratyphi C, S. enteridis, S. infantis, S. haardt, and S. typhimurium, which was similar to other finding (26). Isolation of invasive serotypes such as S. typhimurium indicates the public health significance and may pose health hazards, especially if chicken is consumed undercooked or cross-contamination occurs in kitchen during the meal preparation. The dominated serotype was S. thampson, which might be due to cross-contamination during product handling and distribution. Infection with S. thampson causes diarrhea, nausea and vomiting in humans as well as several diseases in immune-compromised individuals (33).
Contamination rates of the unpackaged samples with different serotypes of Salmonella spp. were higher compared with the packaged ones in both chicken and beef samples. Moreover, Salmonella serotypes were detected significantly higher in the unpackaged chicken compared with unpackaged beef; this might be due to specific physiological features of the beef tissue. We detected Salmonella serotypes more frequently during June, July, September and August, the hottest months of the year in Tehran. Previous studies also showed higher levels of Salmonella serotypes detection during summer (34).
