Intermediate 
A subway system has billions of inhabitants: the bacteria of Swiss cheese and kimchi, bubonic plague and drug-proof bugs and human skin. Now, for the  rst time, scientists have started to catalogue and map the bacteria in a citys subway  and they have found many interesting results.
Dr Christopher Mason, a geneticist at Weill Cornell Medical College, led a team that, for 18 months, tested the New York City subway system for the microscopic life forms that cover its turnstiles, seats, ticket booths and stations. His team found meningitis at Times Square, a trace of anthrax on the handhold of a train carriage and bacteria that cause bubonic plague on a rubbish bin and ticket machine at stations in uptown Manhattan.
The team have strongly downplayed the  ndings of plague and anthrax. They say that there is only an extremely small trace of the latter, that rats likely carried the former and that no one has fallen ill with plague in or around New York for years.
The results do not suggest that plague or anthrax is prevalent, the study says. Nor do they suggest that New York residents are at risk.
In fact, most of the bacteria identi ed by the team are either harmless to humans or bene cial in the citys thriving world of microorganisms.
Some of the results were expected and should be a gentle reminder for people to wash their hands, Mason said. He also said that they found many bacteria of the same sort as those that are bene cial and helpful, like the one used for making cheese.
Bacteria appeared to re ect the eating habits of various neighbourhoods. All around the subway, bacteria associated with cheeses  brie, cheddar, parmesan and the mozzarella found on New York pizza  appeared. The distinctive bacteria of Swiss cheese were more limited to midtown Manhattan and the  nancial district, and the bacteria used to ferment cabbage for kimchi and sauerkraut showed up in the  nancial district and Bay Ridge.
Bacteria associated with illness and infections were extremely common. Species that cause diarrhoea and nausea, as well as E.coli, and the bacteria that can cause skin infections and urinary-tract infections were common all over the city. The species that produces tetanus appeared in Soho and bacteria that cause dysentery appeared at a station in the Bronx and another in Harlem.
Mason and his team collected more than 1,000 samples at all of New Yorks 466 open subway stations. They put the organic materials through a DNA sequencer and, then, through a supercomputer. They identi ed 15,152 distinct species, nearly half of which were bacteria.
The good news, the researchers said, is that these potentially infectious bacteria are not spreading sickness or disease throughout New York. They seem to be normal co-habitants of a city.
In short, the researchers conclude, the subway and city are about as safe as everyone thought.
Mason said people should not be concerned about getting urinary-tract infections from subway seats. You should wash your hands, he said, and probably get some sleep, eat salads and go to the gym, and thats about the same today as it was yesterday.
In fact, he added, Ive become much more con dent riding the subway.
Many  findings made sense: stations like Grand Central and Times Square, where there are more people, had more bacteria and more diversity among them. The Bronx, with its diverse neighbourhoods and stations, had the greatest diversity of bacteria; Staten Island, with just three stops, had the lowest.
The researchers found marine bacteria at South Ferry, a station that  ooded during Hurricane Sandy  but they were surprised to note the species included some normally associated with Antarctica and  fish.
The next steps, Mason said, are studies of other cities, which have begun in Paris, Sa_o Paolo and Shanghai, and continued studies of New York, for instance to see how the results change with the seasons. He said he hoped the research would provide a baseline of research for health of cials and geneticists, and could help health of cials to prevent and track diseases and pathogens.