Staphylococcus carnosus
Also known as S. carnosus
The principal commercial staphylococcus starter, named for meat, whose strong nitrate reductase drives the cured colour and whose clean, well-studied character has made it the model organism of its group.
Staphylococcus carnosus takes its species name from the Latin caro, carnis, flesh: the staphylococcus of meat. Like its partner S. xylosus it is a coagulase-negative, catalase-positive, salt-tolerant, cluster-forming coccus, and it is apathogenic, harmless, sharply distinct from the coagulase-positive pathogen S. aureus. It is a genome apart in a literal sense: of all the sequenced staphylococci its DNA carries the highest GC content, around 34.6 per cent, which sets it cleanly aside from the disease-causing species and marks how far it has specialised to its benign, meaty niche. The species divides into two subspecies, carnosus and utilis.
It came late to its own name. Though staphylococci had been curing meat for as long as controlled fermentation existed, it was only in 1982 that Schleifer and Fischer formally described Staphylococcus carnosus as a species, whereupon a systematic re-examination of dry-sausage starters revealed that most of the organisms long called micrococci were in truth this staphylococcus. Since then it has become, with xylosus, one of the two staphylococcal species used as meat starters worldwide, and, being clean, safe and well-behaved, the laboratory's model organism for the whole group, studied down to its genome and even engineered as a platform for displaying proteins.
Its central gift is colour. It carries a strong, well-characterised nitrate reductase, encoded by the nar operon and switched on as oxygen falls by a sensing system called NreBC, which reduces the nitrate of a Cure #2 or a Salvianda to nitrite; that nitrite, by way of nitric oxide, binds myoglobin into the red nitrosomyoglobin of cured meat. It reduces nitrite further as well, builds aroma through the breakdown of amino acids and pyruvate, and its catalase guards the fat against rancidity, while its acid tolerance lets it keep working alongside the lactic bacteria as they drop the pH.
In a starter it is the colour-and-flavour half of the partnership, added with an acidifying lactic bacterium and, in many blends, paired with xylosus too, since two staphylococci together give a deeper, more stable colour and a rounder aroma. It is chosen above all for reliability, strong and consistent nitrate reduction with a low tendency to off-flavours or biogenic amines, and it is handled like any starter, freeze-dried or frozen, rehydrated and mixed through the meat. It earns its place most in nitrate-cured and long-ripened products.