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Wikipedia describes how antibiotics are produced in ants:

"Metapleural glands ... are responsible for the production of an antibiotic fluid that then collects in a reservoir ... also referred to as the bulla ... From the bulla, ants can groom the secretion onto the surface of their exoskeleton. This helps to prevent the growth of bacteria and fungal spores on the ants and inside their nest."

So how are ants often cited as an important problem in hospitals, spreading infections from garbage and through the patients? Shouldn't their natural antibiotics prevent that?

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    $\begingroup$ Who says that ants are spreading infection from garbage to patients? Do you have a reference for that? $\endgroup$ – Chris Dec 26 '14 at 17:48
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    $\begingroup$ No, that's just a truism that I heard sometimes. $\endgroup$ – Rodrigo Dec 26 '14 at 18:08
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    $\begingroup$ A quick google at "ants hospital infection" shows a lot of scientific papers... $\endgroup$ – Rodrigo Dec 26 '14 at 18:28
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    $\begingroup$ @anongoodnurse A truism is something that is so obviously true that you don't need to justify it. I suspect Rodrigo meant to use some other word. $\endgroup$ – David Richerby Dec 27 '14 at 0:50
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    $\begingroup$ Human beings also have many glands which secrete antibiotic fluid, including our tear ducts. Yet humans are certainly an important source of infectious bacteria. $\endgroup$ – tel Dec 31 '14 at 0:17
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I have worked in hospitals (US) most of my life, treating both community-acquired, and more pertinently to this question, nosocomial (hospital acquired) infections, and have read many articles on the subject.

I have never, ever seen ants mentioned anywhere.

People, flies, cockroaches and rats, yes.

Ants, no.

However, ants are vectors in a few foreign studies. The key word in these studies is potential.

As you stated,

From the bulla, ants can groom the secretion onto the surface of their exoskeleton. This helps to prevent the growth of bacteria and fungal spores on the ants and inside their nest.

No antibiotic covers all organisms. Also, the secretions are to protect the ants against infection, not against the carrier state.

Pathogenic bacteria is cultured from ants themselves. The ant can be thought of as similar to a human: humans make antibodies to pathogens but are vectors of many diseases. Ants protect themselves via secretions, but are hosts to potential pathogens.

Part I: Review of Scientific Data Regarding Transmission of Infectious Agents in Healthcare Settings
Hospital hygiene and infection control
Ants associated with pathogenic microorganisms in brazilian hospitals: attention to a silent vector
Ants in a hospital environment and their potential as mechanical bacterial vectors Insect/Bacteria Association and Nosocomial Infection

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  • $\begingroup$ I agree with that - I have been a lot to hospitals lately and not noticed any ants. $\endgroup$ – Chris Dec 26 '14 at 19:50
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Ants can and do carry loads that are several times their own weight. I grew up in an area with a lot of ants, and a common scene was a long trail of ants acting as a food supply line. Once a morsel is located, they create a long feremone trail to that morsel. A large clump of ants is always working to break the food into smaller pieces, and several ants are usually on their way back to the colony carrying large crumbs. Often these crumbs are the first thing you notice, rather than the actual ants, because they are often so much larger than the ants themselves.

In a hospital, most of the food that ants would scavenge would have fallen from the plate of a human being. Many of those human beings would be carriers of disease. I would expect that the trail that these ants walk is littered with plenty of crumbs-of-crumbs that fall off of their load along the way. If these were large enough for us to see, they would get the attention of the ants and they would carry them back to their colony. But a particle of the microscopic crumb dust they do leave behind might carry several specimens of the germ that sickened some messy patient, even if the ant that dropped it was relatively sterile.

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Other than the fact that ants carry bacteria on their food particles, there could also be alternative reasons that they can spread infections.

Just like antibiotics derived from bacteria and fungi have caused resistant pathogen strains to evolve (MRSA for example), it could also be the case that the bacteria naturally living around the ants have evolved some degree of resistance to the antibiotics, perhaps by effluxing sufficient amounts of them by overexpressing pump proteins to the levels that the antibiotics are only mildly bacteriostatic instead of bacteriocidal. These bacteria can then fall off the ants while in the hospital and lead to disease.

It is reasonable to assume that this could evolve, since the ants frequently communicate socially with one another, and any resistant bacteria or fungi can then quickly colonise a previously empty niche, thus creating significant evolutionary pressure for this.

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