INTRODUCTION

Bacteria have been shown to exist predominantly in nature as sessile populations attached to surfaces in contact with water (Geesey et al., 1977). The numbers of bacteria attached to surfaces have been estimated to be between 1000 and 10000 times greater than the numbers of planktonic bacteria in any given environment (Watkins & Costerton, 1984).

The advantages to the micro-organisms of being attached to a surface have been largely attributed to enhanced scavenging of nutrients from both the bulk water and from the substratum with which the bacteria are associated (Marshall, 1976). Microorganisms at surfaces are considered to be exposed to higher nutrient conditions than their planktonic counterparts and, therefore, are at an advantage in the attached state. As a demonstration of this, Kjelleberg et al. (1982) exposed starved planktonic and attached marine bacteria to 2 mg l−1 of both yeast extract and tryptone. The attached population was able to grow under low nutrient conditions, while the planktonic population was not. Further investigations revealed that when adsorbed to surfaces, both low (Power & Marshall, 1988) and high (Samuelsson & Kirchman, 1990) molecular mass compounds were available for microbial growth. These and other studies strengthened the idea that differences in growth rates of surface-associated bacteria and planktonic bacteria were primarily due to differences in nutrient availability. Thus attached bacteria, while nutritionally at an advantage over their planktonic counterparts, have for many years been considered to be similar in other respects to non-attached bacteria.

Continued research began to expose differences between planktonic and attached bacteria that implied physiological alterations following attachment to a surface.