Science: Mystery of the elusive slime bug: There's a lot we don't know about M. parvicella, a microbe that appears to hinder as well as help the sewage disposal process. Bernard Dixon reports

The engineers who operate one of the most widely used sewage disposal systems have totally inadequate information about the process they are seeking to control. That disquieting statement is made in a paper, published this month. Its authors come from Wessex Water Services in Poole, Dorset, the School of Civil Engineering at the University of Birmingham, and the Czech Drug Research Institute.

Their claim stems from growing concern about a microbe known as Microthrix parvicella. Normally, the possession of a formal name of this sort indicates that we understand the nature of a particular micro-organism, its niche in the microbial world and its characteristic activities. But M. parvicella is tantalisingly different. It can change its shape and behaviour, it is not always easy to grow in the laboratory, and even its true identity is elusive. Yet this so-called filamentous bacterium apparently both enhances and impairs the activated sludge process that breaks down domestic and industrial wastes for billions of people, every day, throughout the world.

Biotechnology - the use of living cells to make useful products and to effect biochemical processes - is relatively new as a term and as an industry. However, by far the largest segment of this industry is sewage disposal, which has a much longer history. From the smallest septic tanks, serving up to 300 people in small communities, up to the activated sludge system that can deal with effluent from populations of two million, the process depends on the combined activities of different types of microbe, living and working together.

In the activated sludge system the sewage is mixed and aerated in a large tank. Slime-forming bacteria grow and form flocs, to which protozoa and other animal-like forms of microscopic life become attached. Filamentous bacteria and fungi are usually present, too. The entire rich community (containing some 10 billion bacteria per gram of sludge) breaks down the effluent continuously. The sludge gradually passes into a further tank where the flocs settle out, leaving clear effluent to be drawn off at the top. Some of the flocculated material is returned to the first tank to maintain the microbial population there, and the remainder is dried and sold as fertiliser.

M. parvicella, first isolated in the Netherlands, is one of the microbes found in activated sludge. Normally, it seems to play a useful role alongside other members of the microbial population. If it becomes a dominant species, however, the same organism prevents the sludge from settling properly, and can also form a type of foam that further impairs the process.

For these and other reasons, sewage microbiologists would like to know a good deal more about M. parvicella and how it functions. Given that understanding, they should be able to manipulate the activated sludge process to prevent the deleterious effects and ensure a more consistent performance.

Before the recent research by Richard Foot at Wessex Water, together with colleagues elsewhere, the shape of M. parvicella was usually described as long, coiled filaments. Over the past two years, however, evidence has emerged to suggest that it might occur in an alternative guise, and that it varies in its responses to staining with dyes (one way in which bacteria are classified into different categories).

Now it appears that M. parvicella occurs in several different forms, only one of which has been reported previously, and that even its correct identity is far from clear. Richard Foot and his collaborators set out to isolate the bacterium by adding material from sewage plants operated by Wessex Water to a type of nutrient medium used by previous researchers to grow the organism.

As they report in this month's Journal of Applied Bacteriology (vol 76, p 301), they failed - on five separate occasions. So they tried instead to separate the microbe physically from sewage flocs under a microscope, using delicate micromanipulators similar to those with which surgeons repair tiny blood vessels. It worked. They managed to remove a single filament of what looked like M. parvicella, which they were then able to culture on nutrient medium.

However, the cultures soon showed that the bacterium appeared in at least three different guises - as long filaments, short filaments and as spore-like structures - which differed in their susceptibility to stains. It is this variablity that has prompted Foot and his colleagues to ask some uncomfortable questions about the true idendity of M. parvicella. Is the bacterium as described in reports from France, South Africa and Australia really the same as that first reported in Holland? Has M. parvicella been wrongly named altogether? Does it actually not belong to the genus Microthrix at all? Is it, as the changes in appearance may suggest, really a species of Nocardia (a group of bacteria that causes pneumonia in patients with impaired immune defences)?

Sewage disposal is a natural process, facilitated by mixed and relatively undefined associations of microbes that also break down dead and waste materials in the environment. Though controlled like any other industrial activity, it is far less precise than, say, the manufacture of pencillin. It works, and with spectacular efficiency, so there has never been a great need to comprehend and describe the process in exhaustive detail. Nevertheless, the lack of understanding revealed by this (attempted) study of M. parvicella is quite extraordinary. Can we afford to live with such ignorance?

(Photograph omitted)