Health and Hokey-pokey

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Andrew Caldwell

Doctors may soon be prescribing ice cream for sore throats if a recent antibiotic breakthrough goes into production.

It has long been recog­nised that traditional antibiotics, one of the great advances of the 20th century, are losing their effectiveness through over-prescription and misuse. Bacterial populations which were once wonderfully susceptible to a few modest capsules of penicillin now munch through the drug with impunity.

Although augmentin—­penicillin combined with a compound which inhibits the main enzyme used by penicillin-resistant bacteria to degrade the drug—has given penicillin a second wind, and new types of antibiotics, such as the cephalosporins, have been produced, resistance to these is also becoming widespread. The basic problem is that bacteria are developing resistance to these chemical weapons more rapidly than science can forge new ones. There is an urgent need for fresh antibiotics, and for drugs which have entirely new modes of antibiotic action.

But finding or producing such compounds is not easy. To minimise the possibility of side effects, the ideal drug should target a biochemical pathway that is vital to bacteria but absent from mammals, and since the biochemical underpinnings of life differ little even between such disparate groups, progress is slow.

Now a research group headed by University of Otago microbiologist John Tagg has discovered a new antibiotic compound that shows sufficient promise for the university to have filed international patent applications and for a new company to have been established to commercialise the discovery. The new compound, salivaricin B, has been isolated from a harmless bacterium, Streptococcus salivarius, which occurs naturally in the human mouth—especially on the tongue—and is able to inhibit the growth of the related pathogen Streptococcus pyogenes plus some other streptococci and micrococci.

Tagg’s general approach has been to study the ecology of bacteria living in the mouth. As early as 1877, Louis Pasteur noted that the growth of common, benign bacteria could interfere with the growth of pathogenic bacteria, and wondered whether this competition could be exploited for therapeutic purposes. Tagg and his group suspected that some individuals were less likely to develop “strep throat” because their naturally occurring oral bacteria effectively inhibited invading pathogens.

One of the tactics the “tangata whenua” bacteria use against invaders is the secretion of antibiotic peptides known as bacteri­ocin-like inhibitory substances (BUIS). Salivaricin B, a 25-amino acid peptide (a small protein), is one such substance. It contains several unusually modified amino acids which endow it with a peculiar structure compared to most peptides, and it is termed a lantibiotic. It is not the first lantibiotic to be discovered, but it is the first with direct application to human health. Nisin, produced by bacteria that live in milk, is another member of the family, one that is used in the dairy industry as a natural food preservative.

In people who get fewer throat infections, a high percentage of the resident S. salivarius secrete salivaricins; in those more susceptible to infection, few or none secrete these antibiotics.

If it becomes commercially available, salivaricin B will not be the sort of antibi­otic one takes as a prescribed course of pills. Rather, the idea is to introduce a salivaricin B-secreting strain of bacterium into peoples’ mouths, and leave it growing there. It will be a preventative rather than a therapeutic measure.

A bonus is that the antibiotic also kills Streptococcus sobrinus, a species which causes tooth decay. And there is no evidence yet that susceptible bacterial species can develop resistance to it.

Among the questions still to be answered is how to ensure that the new strain of bacterium remains in residence for a long time. Tests to date indicate that in some individuals the beneficial infections will last for months, but in others only days.

And how will one acquire these useful infections? Probably by eating milk or ice cream that has been laced with the bacterium. Compli­ance is not expected to be a problem.

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