BOSTON (FRONTLINE MEDICAL NEWS) – Almost half of all sand and water samples taken from Midwestern freshwater public beaches tested positive for Staphylococcus aureus isolates during the summertime, but numbers fell dramatically in the fall and spring.

Overall, according to a study of 10 public beaches in Ohio, almost half of the isolates were resistant to erythromycin, 41% were multidrug resistant, and about 7% were methicillin-resistant S. aureus (MRSA).

In addition, among the 70 S. aureus isolates, 21.4% had the Panton-Valentine leukocidin (PVL) gene, which encodes for a pore-forming toxin identified as a virulence factor in some strains of staphylococcus. The study results were presented by Dipendra Thapaliya, a research associate at Kent State University, Ohio,, during a poster session at the annual meeting of the American Society for Microbiology.

In light of these findings, the study’s senior author, Sara Smith, PhD, professor of epidemiology at the university, said in an interview that beachgoers should take reasonable precautions, including washing lake water and sand off young children upon returning home, and making sure to shower at the beach if facilities are available.

“Staphylococcus does well in a salty environment, but it is a fairly hardy organism,” Dr. Smith said, noting that S. aureus has been found in the sand and water of marine beaches, but it has been less well studied in freshwater environments.

Public beaches in Ohio were the source of the samples. Some sampling was done along the shores of Lake Erie, but smaller inland lakes also were tested, In all, 280 sand and water samples were collected in a 3:1 water to sand ratio.

The samples were incubated and plated for culture and subculture according to established methods, and then tested for S. aureus via catalase and coagulase testing, as well as latex agglutination testing. Isolates were then subjected to antimicrobial susceptibility testing, as well as polymerase chain reaction (PCR) testing for the PVL gene and for the mecA genes found in MRSA. Final isolate identification was achieved by staphylococcal protein A (spa) and multilocus sequence typing.

Of the 27 spa types found from 70 isolates, two common strains, t008 and t002, were the most frequently detected. One livestock-associated strain, t571, was also identified.

Though beaches are frequently contaminated with E. coli and other enteric pathogens, the source is often birds or other wildlife, said Dr. Smith. To try to ascertain the source of the staphylococcus in the summer samples, Mr. Thapaliya and his collaborators repeated testing during the winter months and in the spring, before beachgoers had spent time at the waterside.

Those samples from the months when the beaches were empty of people showed much lower levels of S. aureus: In the summer, S. aureus isolates were found in almost half of the samples obtained (55/120, 45.8%). By contrast, only 5 of the 120 fall samples (4.8%) and 4 of the 40 spring samples (10%) were positive for S. aureus.

“The high prevalence of S. aureus in summer months and presence of human-associated strains may indicate the possible role of human presence in S. aureus contamination in beach water and sand. However, we need further study to confirm such a conclusion,” wrote Mr. Thapaliya and his coauthors.

To that end, Dr. Smith said that another, stronger piece of supporting evidence that the S. aureus does come from “the crush of human bathers” would be to identify and match isolates from individuals who frequent the beaches with isolates found in sand and water. Dr. Smith said that she and her team are currently seeking funding to carry out these next steps.

The investigators reported no relevant disclosures.

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