Instituto Valenciano de Microbiología

Masía El Romeral
Ctra. de Bétera a San Antonio Km. 0.3
46117 Bétera (Valencia)
Phone. 96 169 17 02
Fax 96 169 16 37
CIF B-96337217


Methicillin-resistant Staphylococcus aureus (MRSA) in meat products (meat products):

Culture; molecular detection of MRSA; Molecular typing SCCmec, ST (MLST) and spa.

Staphylococcus aureus is a Gram - positive bacterium, which produces catalase, habitual diner skin and mucous membranes of interest because it can produce enterotoxins causing food and capable of producing many kinds of infections poisoning.

Methicillin-resistant staphylococcus aureus -RM- (SARM = MRSA: Methicillin Resistant Staphylococcus aureus) is the same bacteria that acquired many years resistance to penicillinase resistant penicillins isozasólicas (as methicillin). Penicillins penicillinase resistant were introduced in the pharmacological armamentarium to treat infections by Staphylococcus aureus strains which have become resistant to penicillins by producing penicillinase enzyme. The basis of resistance to this new class of antibiotics ?-lactam resides in the ability to synthesize a new PBP (Penilcillin Binding Protein), one of the enzyme proteins located at the plasma membrane of the bacterium whose function is to contribute to the synthesis of cell wall. The new PBP known as PBP 2a (PBP or 2') has the characteristic that is not the target of any ?-lactam antibiotic, except for a recent generation cephalosporins including ceftaroline and Ceftobiprole. Unable to be inhibited by this PBP-2a ?-lactam antibiotics, you can synthesize cell wall in the presence of them, surviving bacteria after treatment with this group of antibiotics.

Strains of Methicillin Resistant Staphylococcus aureus (MRSA MRSA) initially in hospitals as the place where the selection is favored, thanks to widespread use of antimicrobials. However, later they were meeting outside hospitals in the Community. To have some different characteristics the strains found in the Community with regard to those found in hospitals, CA-MRSA (Community Associatted Methicillin Resistant Staphylococcus auresus) were named to distinguish them from those usually found in hospitals (HA-MRSA: Hospital Associatted Methicillin Resistant Staphylococcus auresus). These two strains differ in some features, essentially strains HA-MRSA often more resistant to other antimicrobial and lack the toxin (leukocidin) Panton-Valentine, while CA-MRSA strains are typically more sensitive to other antimicrobial and many have toxin (leucocidin) of Panton-Valentine. At the same time, the types ST (Sequence types) differentiated by MLST (multi-locus sequence typing) by sequencing of 7 conserved genes and spa types (Staphylococcal Protein A), are usually different in both groups of strains. Currently the CA-MRSA strains pose a global problem causing significant proportion of invasive infections. PFGE (Pulse Field Gel Electrophoresis-Electrophoresis pulsed field) have differentiated 8 types (USA100 to USA800), and of these types USA300 and USA400 are the related CA-MRSA strains, whereas other types are more related strains HA-MRSA. There is another way to differentiate strains of MRSA, by studying a mobile genetic element called SCCmec (Staphylococcal Chromosomal Cassette mecA). SCCmec contains mecA complex, which includes the mecA gene and one or more regulatory genes, and a gene complex comprises a cassette chromosomal recombination (ccr. Cassette chromosome recombinase) that regulates the insertion and excision of the cassette into the bacterial chromosome. So far described complexes eight different mec and 4 cassettes chromosomal recombination. The CA-MRSA strains usually correspond to the type IV SCCmec (Staphylococcal Chromosomal cassete mecA) and typically encode Panton-Valentine leukocidin.

Interest Staphylococcus resistant Staphylococcus aureus in meat

The relationship between Staphylococcus aureus RM and animals began to settle when in 1972 he found in milk from a Belgian cow mastitis affects. Since then, it has been found in many animals: dogs, cats, sheep, chickens, horses, rabbits, seals, psittacine birds (parrots and the like), turtles, bats, guinea pig, chinchilla. Several studies have been conducted to determine the prevalence of S. aureus strains MR in different animals in slaughterhouses and butcher shops, as well as personnel working in contact with animals (farmers, slaughterers, butchers, ...), being prevalences different. Some studies, such as one conducted in France, proved to be a farmer carried a higher risk of MRSA colonization (44 vs. 24%). The prevalences found in several studies differ from country to country and even within the same country. For example in the Netherlands it was found that 39% of pigs brought to slaughterhouses were colonized with MRSA. In U.S.A. was found in 65% (2012); Cases in Germany at 65% turkeys and turkey meat 32% (2014). The prevalence usually higher in turkeys (31%), chicken (27%) and calves (16%). Some studies have found the presence of S. aureus in 42 to 45% of pigs studied and in 25 to 35% of cattle, but of these, only a small percentage were S. aureus RM.

In pigs it has been found preferably untypable strains by PFGE, MLST type ST398 (CC398: Clonal Complex 398) and spa t011, t034, t108, t567, t899, t939 and types. Some of these strains are producers of the Panton-Valentine leukocidin ((PVL). There are some studies they show as pigs that were previously negative for MRSA positive after being treated with some antimicrobials (oxytetracycline), for playback problems. However, these MRSA strains found in pigs raised even released, even with the type ST398, which would suggest that is a clone of the animal itself. in Asia would be the most prevalent type ST9.

In general, these S. aureus strains RM are more like strains HA-MRSA, so it was thought that the animals, at least the company, the acquire through a "humanosis" (contamination from people). The discovery of these strains in animals for meat production in meat from butcher shops, and personnel concerned with animals or handling meat, has raised many considerations regarding the original source of the strains (people animals and / or meat; animals to people). In this sense it has been shown in Europe, in some cases, transmission from pigs to people (farmers and their families) of S. aureus ST398 type RM.

Today it is recognized that SARM has entered the food chain and strains from animals are named LA-MRSA (Methicillin Resistant Livestock Associatted). For these reasons there is concern about possible contamination from the farm, through the holder (Farm to Fork transmission). Despite this generally it is not considered a problem only in certain circumstances, if the meat is properly treated before consumption because the heating would destroy the bacteria, especially because contamination of the meat is superficial. Contamination of food products is a problem for people who work with animals or who work in slaughterhouses or butchers, because they can be colonized by these strains and suffering from a staphylococcal infection caused by methicillin - resistant strain. The same may occur for consumers of raw meat that can be colonized by these strains. Described major hospital outbreaks due to this route of contamination (Erasmus Medical Center in Rotterdam, 1995). In these situations it is usually an immunocompromised patient infected by ingestion that can make a sepsis and even death.

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