April 23, 2017

Monkey MRSA

Our short letter on a macaque-specific clone of MRSA is now available on the Journal of Antimicrobial Chemotherapy (paywall).

The story is quite an interesting one. Three years ago, one of the macaques used in neurosurgical research at the SingHealth Experimental Medicine Centre (SEMC) developed a wound infection (the monkeys went around with what looked like metal caps on their heads). To the great surprise of the vets and the microbiologist, the wound swab grew MRSA (methicillin-resistant Staphylococcus aureus). Having an institutional memory of MRSA among pigs used for diabetes research in 2005, the SEMC team quickly got into work, screening macaques across all their 3 facilities and also the humans that had been in contact with the macaques.

The results were astonishing. All 52 macaques tested were found to be colonized by MRSA! Among the 28 humans, two were also found to be MRSA positive. The question then was whether the humans had passed MRSA to the monkeys, or vice versa. This question is answered nowadays by performing molecular “fingerprinting” on the bacteria to determine whether they are related, and by using tests with slower “molecular clocks” (in the case of S. aureus, this is usually a technique called multi-locus sequence typing or MLST), one can also attempt to determine which part of the world or from what other animal the bacteria arose (assuming that enough people had contributed such information to the global MLST databases).

The microbiologist from SGH quickly determined that the MRSA isolates (even the ones from the two humans) were not native to humans, except for a single macaque MRSA that matched the major MRSA clone in Singapore hospitals – probably acquired from a healthcare worker that had been in contact with the monkey in question.

In the meantime, investigators from the University of Washington described an outbreak of MRSA among the primates and personnel of the Washington National Primate Research Center in 2015 in the Journal of Antimicrobial Chemotherapy. They had found two unique MRSA clones among the primates (and the attending humans), ST188 and ST3268. Fortunately, they had sequenced representative isolates of both clones and had deposited the sequences into a public access genomic database.

When we sequenced representative macaque MRSA isolates from Singapore, courtesy of Prof Matthew Holden’s laboratory at St Andrews University, we found to our surprise after analysis of their entire genomes that not only were our isolates also part of the ST3268 MRSA clone, but the  genomes of the U.S. isolates (TXA and TXB) also fit within the cluster of local isolates. As one might surmise from the “phylogenomic tree” below, the macaque MRSA isolates from Singapore were also not as closely related as one might imagine if there had been a point source outbreak in Singapore, suggesting that many of the monkeys were already carrying MRSA prior to their entry into SEMC.


“Phylogenomic” tree of local macaque MRSA isolates as well as two isolates from the Washington National Primate Center (TXA and TXB).

The SEMC records showed that many (but not all) of the macaques had been imported from Vietnam at various time starting from 2009. None of the monkeys from SEMC had subsequently been exported to the U.S.A. It would be interesting to find out how common this MRSA clone is among the macaque population in Asia. What we did not report in the research letter, but which was nonetheless interesting, was that these macaque MRSA isolates were resistant to a number of non-beta-lactam antibiotics, suggesting that these MRSA might not necessarily be found in wild populations of macaques, but rather only among farmed macaques where antibiotic use might be far more prevalent.

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Antimicrobial resistance, Clinical microbiology, Infectious diseases, MRSA, Outbreak, Singapore


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