Gupta: Tracing anthrax source a good tool

(CNN) -- Doctors in northern Virginia said that preliminary tests indicated no anthrax present in a man who entered a hospital Monday night.

The man has connections to American Media Inc., a Florida-based company that owns the tabloid The Sun, where one worker died from anthrax and another was exposed to it.

Now, authorities are trying to trace the source of the Florida anthrax. CNN's Dr. Sanjay Gupta has been looking into developments in this case.

CNN: How are authorities going about tracing this sort of anthrax?

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GUPTA: There are some good tests out there, and one of the tests that we haven't heard too much about ... is something called PCR, polymerase chain reaction. I bring this up not to be too science-y but rather to say that these tests are very good genetic tests.

They actually can tell us two things. One is, is this anthrax artificial or natural? And two, if it is artificial, from where did it come? This bacteria doesn't exist in very many places around the country so it's relatively easy to try to figure out from where exactly it came.

CNN: So what you're saying then is because it's so rare, you can actually find its source by tracing it back?

GUPTA: That's right, and that's what the experts are telling me. They can find the soil that the anthrax spores existed in before they were manufactured, altered and all those things.

I want to add one more thing though, and I think it's an important point. The first anthrax patient was sensitive to penicillin. That's an important point because typically manufactured anthrax is antibiotic-resistant to penicillin.

CNN: We have all been told how lethal [anthrax] is. Why is [the second patient] still with us? Why is he still alive?

GUPTA: Well, I think it's an important point because many times using anthrax the way that we've been speculating it could have been used isn't very effective. A lot of times, these spores just get caught up in the nasal cavity. They never make their way down to the lungs to actually wreak the havoc. That unfortunately happened in the first gentleman. [But it's] not a very effective tool in the world of bioterrorism.

CNN: The other thing we're looking at -- some reports that possibly the anthrax arrived at this building in Florida by way of a letter. If that is the case, would you have to go back and trace everyone that came into contact and had the possibility to come in contact with the letter right through the postal system?

GUPTA: Well, typically, with these spores … you'd have to breathe in a large number of spores, several thousand spores to actually have any effect. There are some people that may have come in contact, may have gotten a little bit and breathed in a few spores, such as maybe this gentleman who has had the positive nasal swab.

But for the most part, as we're seeing, that's not going to cause any problems. Someone who was able to actually hold it in their hand and then you breathe in several thousand spores -- that would be a different story -- as we maybe saw that with the first gentleman.

CNN: We want to point out the case in Virginia that was first reported Monday night. Tuesday morning, they say there is no connection at this point?

GUPTA: Yes, but you know, in some ways, this is a public health triumph, and I think that's important to point out. Public health officials went down to Florida. They found a guy who hasn't developed any symptoms. He's going to get treated. They found him by a very sensitive test called a nasal swab test. We're probably going to see a lot of false positives around the country, certainly better than the alternative of a false negative.