Fangs for the memory

It was once mandatory in Western movies for villains and the odd innocent cowpuncher to die after being punctured by a rattlesnake.

A bite from a big rattler, especially the eastern diamondback Crotalus adamanteus, which grows to 2.75 metres, was a fast ticket to Boot Hill in the days before anti-venom.

Compared to some Australian snakes, rattlesnake venom is not particularly toxic; predominantly it contains hemotoxins rather than paralysing neurotoxins. Nevertheless, rattlers rank among the world's most dangerous snakes because of the huge amounts of venom they can inject with their formidably long fangs.

Californian microbiologist Diane Citron and her colleague Dr Ellie Goldstein, an infectious diseases specialist at the University of California in Los Angeles, have studied the bacteriology of bite wounds - among other projects - at Citron's base, the R.M. Alden Research Laboratory in Santa Monica, for many years.

They have found that, in addition to the hemotoxins present in rattler venom, a bite can also deliver an impressive list of aerobic and anaerobic bacteria.

In subsequent studies they also found that rattler venom also contains a number of potent bactericidal compounds capable of rapidly killing antibiotic-resistant strains of species like Staphylococcus aureus, so anyone surviving a rattler bit might be lucky enough to avoid a nasty bacterial infection.

Several years ago Goldstein, who regularly co-authors research papers with Citron, rang her collaborator with an unusual offer.

A young man who had visited the emergency clinic several times for treatment for bites from his pet snake had finally concluded that life in close proximity to an ill-tempered rattlesnake was an unnecessary hazard. Smartly, he donated the reptile to Goldstein and to science.

Previous experience with snakebites had revealed an absence of infection, and Citron was intrigued by the opportunity to examine the late serpent's mouthparts for bacteria.

Citron and Goldstein carefully dissected the rattler's mouth, and aspirated or swabbed all parts, then cultured the samples on a variety of agar media. Curiously, the colonies that grew on their agar plates proved to be typical of the anaerobic gut flora of mammals, including Fusobacterium, Clostridium, Bacteroides, Salmonella, Enterobacter, Serratia, Klebsiella and the inevitable E. coli.

What on Earth were microbes from the distal end of a mammalian gut doing at the proximal end of a reptile?

The penny dropped. The snake ingests its mammalian prey head first, compressing its abdomen, thus ejecting a sample of the prey's enteric flora into the snake's mouth to be passed on to the next victim. Fangs for the memory...

Bite-wound infections

Citron's bacterial bailiwick is not limited to companion animals: it encompasses the oral floras of any beast that chances to fang an unwary human being.

Citron is an invited speaker at the South Pacific Congress in Auckland this week and will give a talk on bite-wound infections. "With bite wounds, whether it is by a human or an animal, there are a couple of different components to the trauma," she says.

"There is traumatic punctures and tears of the tissues, and often, with large dog bites, there are crushing injuries.

"Then you have the potential impact of the plethora of anaerobic bacteria in the animal's saliva and tooth plaque introduced into the injured tissues. These can cause very nasty infections."

Citron says she has also seen one case of a bacterial infection from a pig bite. "These are different kinds of animals, and the bite really caused a lot of inflammation, which required the patient to be hospitalised and put on intravenous antibiotics."

Her colleagues at the University of California at Davis have analysed the oral floras of marine mammals like Orca killer whales, and sea lions that occasionally bite incautious beachgoers along the Californian coast.

Instead of hosting anaerobic marine bacteria such as Vibrio - the agent of cholera - marine mammals also have anaerobic bacteria very similar to the oral anaerobes of terrestrial mammals. Citron has cultured samples from several seal bites inflicted on swimmers at local beaches, but they yielded no anaerobes.

She begins the identification process by plating organisms onto standard selective and differential media. "You start with conventional techniques to identify common species, then, if they don't work, you use more complex techniques.

"Beyond that, you extract DNA for 16S ribosomal gene sequence analysis. When you get the sequence data, you go onto the internet to search for matching sequences.

"There's a huge diversity of organisms now, and there's a lot of sequence overlap between species or strains."

Workshop on species identification

Also at the conference, Citron will run a day-long workshop on culture and identification of anaerobic bacteria from clinical infections, from phenotyping to molecular genotyping.

She says the number of bacterial cells in the human gut outnumber the total number of cells in the human body by a ratio of 10:1.

"Anaerobic infections typically involve a break in the normal mucosal barriers lining the oral cavity, or other mucosal surfaces, allowing the bacteria to invade the surrounding tissues and sometimes the bloodstream.

"The same, friendly bacteria that make vitamins in the human gut can take over in the bloodstream and make you very ill. They can secrete toxins or enzymes like collagenases and haemolysins, that break down tissues and cause abscesses, gas gangrene and bacteraemias.

"If they get into the wrong place, staphylococci from the skin, or enterococci from the gut, can cause nasty, opportunistic infections. If your immune system is compromised, the harmless, probiotic lactobacilli in yoghurt can get into the bloodstream and cause bacteraemia.

"Enteric clostridia can cause gas gangrene in limbs, requiring amputation of affected tissues. Many soldiers fighting in the trenches in World War 1 also developed gangrene but from exogenous sources.

"They were living and defecating in trenches, and clostridia from human and animal excrement were contaminating wounds and causing gas gangrene, and ghastly necrotic infections of muscles.

"Even today, cancer patients who have had surgery, and whose immunity is compromised because of chemotherapy or radiotherapy can develop bacteraemia if anaerobic bacteria normally resident in the oral cavity or colon find their way into traumatized tissue.

"I've seen a dramatic case of bacterial meningitis in a teenager, originating from a retropharyngeal abscess caused by Fusobacterium necrophorum."

She says patients with periodontal disease or mucositis can also develop infections when anaerobic bacteria from the oral mucosa invade the bloodstream.

"I had another very interesting case of a woman who developed a Fusobacterium infection of the bloodstream because of mucositis resulting from chemotherapy for an underlying, haemotologic malignancy.

"The particular strain was resistant to penicillin, so she did not respond to treatment with ampicillin. Unfortunately, she died from a seemingly innocuous organism that just ended up in the wrong place."