New worlds to explore

by Bernard Dixon

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http://infection.thelancet.com Vol 6 February 2006 73

It is a century since microbiologists experienced such a sense of adventure. Then, the early microbe hunters were incriminating one organism after another as specific causes of human, animal, and plant diseases. Others were isolating non-pathogens as agents of change in the natural environment. Now, as molecular methods augment and supplant classical techniques, revolution is afoot once more. Throughout all sectors of microbiology, pure and applied, new worlds are being explored and old problems addressed with greater precision.

Last month I wrote about the use of PCR to characterise the microbial flora of aeroplanes. This month comes news of researchers harnessing fluorescence in-situ hybridisation to clarify the possible involvement of the gut flora in autistic disorders in children.

The notion behind this work has been around for several years, triggered in part by clinicians who have discovered that many autistic children have received several courses of antimicrobial therapy. There has also been one report that vancomycin, given orally, can lead to significant short-term improvements in patients receiving this antibiotic (J Child Neurol 2000; 15: 429). Could antibiotic treatment have modified the children’s intestinal microflora in such a way as to provoke the developmental problems that characterise autism?

Efforts to go beyond this anecdotal evidence have proved frustrating. Indeed, the complexity of autism has bedevilled investigations into a much wider range of possible causes. Autism is not a single condition but a spectrum of abnormalities affecting a child’s social, communicative, and imaginative development. Known as autistic spectrum disorders (ASDs), they are four times commoner in boys than girls.

There is limited evidence that gluten-free and possibly other exclusion diets can help—in two ways. First, some reports suggest that the behaviour of youngsters consuming diets of this sort improves in consequence. Second, removal of nutrients such as gluten or casein can lead to a lessening of the gastrointestinal symptoms (abdominal pain, bloating, and diarrhoea or constipation) that affect many autistic children.

It is possible that these two effects are closely linked. However, further understanding has awaited detailed comparisons of the composition of the gut flora of people with ASD as compared with healthy controls. A few authors have written about the likely role of clostridia, for example, which can generate powerful neurotoxins. But there have been no persuasive studies to substantiate the influence of this or any other group of organisms.

Anne McCartney, Helena Parracho, and colleagues at the University of Reading have now made an important advance in our understanding. They investigated 48 boys and ten girls (aged between 3 and 16), all of whom had been diagnosed with ASDs, and compared their intestinal flora with that of control individuals. Instead of culture methods, the researchers used fluorescence in-situ hybridisation with oligonucleotide probes targeting predominant components of the intestinal flora.

The results, published in the Journal of Medical Microbiology (2005; 54: 987), show a significantly higher incidence of Clostridium histolyticum in the faecal flora of ASD patients than in healthy children. There were no significant differences between the two groups in total bacterial loads, nor in the incidence of bifidobacteria or other potentially relevant organisms.

“The numerically predominant bacterial population in samples from ASD patients was C histolyticum
(Clostridium clusters 1 and 11)”, the authors write. They have never previously observed this type of phenomenon when investigating the intestinal microbiology of human subjects suffering from conditions such as ulcerative colitis, bowel cancer, or irritable bowel syndrome. “These results support the hypothesis of an association between clostridia and the development of certain autistic characteristics.”

Whether the association is to any degree causal remains an open question. However, the Reading findings are impressive, not least because molecular approaches have enormous advantages over traditional culture techniques. Even so-called selective methods are imperfect in recovering relevant organisms, as well as being subject to problems arising through the transport and storage of specimens.

The revolution gathers pace.