One evening, as the sun was setting over the beach, Walter the warty sea cucumber (although he preferred his grander name, Parastichopus parvimensis) was enjoying his usual dinner of sand. From in between the grains, he selected juicy, crunchy, and delicious meifoauna, delighting in the variety of flavours. But, unfortunately for Walter, among them, Nemo the nematode had just finished his dinner, and that had included some scrumptious morsels of Maribacter polysiphoniae, a very uncultured microbe. Now, Walter hated the taste of Maribacter polysiphoniae, and attempted to spit out his mouthful. But, being a warty sea cucumber, Walter couldn’t spit….
Well, perhaps there’s a good reason why I haven’t tried my hand at writing children’s stories. But I did write (unbelievably, just over two years ago) about meiofauna, the staggering diversity of miniature critters that live in the spaces between sand grains. Alarming as the idea of a zoo in your sandcastle might be, I commented how grateful we should be to the meiofauna, for they graze on bacteria and so keep our beaches clean. But what, exactly, are these bacteria? Well, at the head of this post is a portrait of a small sample on the surface of a sand grain. This extraordinary scanning electron microscope image comes from the work of Anthony D'Onofrio and his colleagues at the Antimicrobial Discovery Center of Northeastern University in Boston. This particular study has as its focus the bacterial life on intertidal sand grains from Canoe Beach in Nahant, Massachusetts, and, scrutiny at successively higher magnifications reveals this (taken from the paper by D’Onofrio and others available online):
What we are looking at are incredible communities of bacteria – but these are not just any communities, since they have united together to cling to each grain as a biofilm, and biofilms are startling and mysterious forms of life. Isolated bacteria, floating around in the seawater, are referred to as planktonic, but when they alight on the surface of a grain of sand and team up, something amazing happens: the whole becomes greater than – and very different from – the sum of the parts.
Now I, for one, did not know much about biofilms until I came across these amazing images a while ago. But I should have done, since I carry them around continuously, and encounter them frequently. My quest for further understanding of these constant companions led me to the superbly informative website of the Center for Biofilm Engineering at Montana State University. Herewith, from their “Biofilm basics” section:
You may not be familiar with the term "biofilm," but you have certainly encountered biofilm on a regular basis. The plaque that forms on your teeth and causes tooth decay is one type of bacterial biofilm. The "gunk" that clogs your drains is also biofilm. If you have ever walked in a stream or river, you may have slipped on biofilm-coated rocks.
Biofilm forms when bacteria adhere to surfaces in moist environments by excreting a slimy, glue-like substance. Sites for biofilm formation include all kinds of surfaces: natural materials above and below ground, metals, plastics, medical implant materials—even plant and body tissue. Wherever you find a combination of moisture, nutrients and a surface, you are likely to find biofilm.
A biofilm community can be formed by a single bacterial species, but in nature biofilms almost always consist of rich mixtures of many species of bacteria, as well as fungi, algae, yeasts, protozoa, other microorganisms, debris and corrosion products. Over 500 bacterial species have been identified in typical dental plaque biofilms. Biofilms are held together by sugary molecular strands, collectively termed "extracellular polymeric substances" or "EPS." The cells produce EPS and are held together by these strands, allowing them to develop complex three-dimensional, resilient, attached communities. Biofilms can be as thin as a few cell layers or many inches thick, depending on environmental conditions.
And here’s their illustration of the biofilm life cycle (perhaps “bio-horror film” might be a more appropriate name – this is going on in my mouth?):
Biofilms are, quite literally, everywhere, and they present huge puzzles and mysteries. But understanding exactly what they are and how they work is vital. Efforts to combat bacteria focus on their planktonic manifestations, but biofilms are different, and are more resistant to attack than their free-floating versions. And what is more, the traditional approach to developing antibacterial weapons and strategies is to culture the critters in the lab and experiment on them. Pluck a bunch of bacteria from the environment and 99% of them will prove to be “unculturable” – they simply won’t grow in the lab. So what makes them grow in nature? This is where the work of D’Onofrio and his colleagues comes in – they are identifying the growth factors produced by culturable bacteria that trigger growth in their previously unculturable relatives. It would seem to be an extremely complex relationship in which some members of a biofilm community are intrinsically critical for the growth of others – it’s one big co-operative.
This has to be related to another extraordinary characteristic of biofilms: their DNA expression – which genes are switched on and producing proteins – is different from that of the same group of bacteria when they are separate; a biofilm is a different kind of organism in its own right. It’s no wonder that the Center for Biofilm Engineering describes how
The study of biofilms has skyrocketed in recent years due to increased awareness of the pervasiveness and impact of biofilms on natural and industrial systems, as well as human health. Biofilms cost the U.S. literally billions of dollars every year in energy losses, equipment damage, product contamination and medical infections. But biofilms also offer huge potential for bioremediating hazardous waste sites, biofiltering municipal and industrial water and wastewater, and forming biobarriers to protect soil and groundwater from contamination. The complexity of biofilm activity and behavior requires research contributions from many disciplines such as biochemistry, engineering, mathematics and microbiology. New insights into the mysteries of biofilm are being published daily in a wide variety of science and engineering journals.
But there are always two sides to every coin, and the story of bacteria and sand is no exception. The prediliction of planktonic bacteria for alighting on a grain of sand and massing themselves into a biofilm is the basis for slow sand filters. Originally developed in the UK in the nineteenth century, and influential in the eradication of cholera, these water filtration systems continue to be used for metropolitan water supplies and, in modern refinements, on a small scale in developing countries. The basic principle is simple: a biofilm forms on the surface of a bed of sand and continues to attract micro-organisms from the passing water. The biofilm or Schmutzdecke (another wonderful word) builds up significantly, removing up to 99% of the bacteria from the water – but, of course, eventually the sand must be cleaned. This technology is now very sophisticated, and so, although we have yet to understand how it all works, we should be grateful for the attraction of the surface of a grain of sand to passing bacteria.
[The extraordinary scanning electron microscope images are reproduced here under a Creative Commons Attribution 2.5 License, courtesy of the Lewis Lab at Northeastern University. Images created by Anthony D'Onofrio, William H. Fowle, Eric J. Stewart and Kim Lewis.]