Identification and study of the behavior of S. aureus and S. epidermidis in fresh and frozen strawberries.

BACKGROUND: Methicillin-Resistant Staphylococcus aureus (MRSA), which is resistant to many antibiotics, creates a serious problem for human health if present in food. This study aimed to assess the quality of commercially-available fresh and frozen strawberries and to compare the behavior of staphylococci in these fruits as affected by the temperature of freezing.

METHODS: The research material included different species of fresh strawberries and strawberries frozen with the fluidization method to –40°C and packaged in an industrial environment. These were checked for the presence of Staphylococcus aureus resistant to methicillin (MRSA). The strawberries were purchased at food markets in the Tricity and were divided into two groups (A and B). Group A was analyzed without washing, group B was washed in sterile, distilled water for 15 minutes. The strawberries were placed in sterile PE/PA bags. Then 1 mL of Staphylococcus aureus Rosenbach ATCC 25923 and Staphylococcus epi- dermidis ATCC 12228 of known inoculum were added to each bag (except the control samples). The samples were mixed thoroughly and then hermetically sealed. The samples were then frozen and stored in the freezer at a temperature of –18 ±2°C for 2 months. In the material being tested Staphylococcus aureus was cultured in selective Baird-Parker RPF Agar. Incubation was carried out at a temperature of 37°C for 48 hours in sterile conditions. After 48 hours of incubation, characteristic colonies were transferred onto the reaction field of the PROLEXTM STAPH XTRA LATEX KIT.

RESULTS: The results obtained show that 1/3 of the samples of commercial strawberries analyzed were colo- nized by methicillin-resistant Staphylococcus aureus (MRSA). The process of fruit washing was observed to reduce the number of samples containing MRSA from 11.7 to 8.3%. There was no significant difference in the size of the S. aureus ATCC 25923 population after freezing the strawberries at –18°C, depending on the particular washing process for these fruits. The analysis of strawberries frozen with the fluidization method at –40°C showed a minimum contamination degree with S. aureus after the period of storage.

CONCLUSIONS: Studies have shown that MRSA are present in 15.4% of strawberries obtained from the field. Storing strawberries frozen at –18ºC causes a reduction in the number of S. aureus by 0.16 log10CFU/g  and S. epidermidis by 0.47 log10CFU/g when they were subjected to rinsing after harvesting. Effective inhibition of MRSA in strawberries is obtained when fluidization technology is applied at –40ºC.