Role of bacterial adhesion in the microbial ecology of biofilms in cooling tower systems

Abstract

The fate of the three heterotrophic biofilm forming bacteria, Pseudomonas aeruginosa, Klebsiella pneumoniae and Flavobacterium sp. in pilot scale cooling towers was evaluated both by observing the persistence of each species in the recirculating water and the formation of biofilms on steel coupons placed in each cooling tower water reservoir. Two different cooling tower experiments were performed: a short-term study (6 days) to observe the initial bacterial colonization of the cooling tower, and a long-term study (3 months) to observe the ecological dynamics with repeated introduction of the test strains. An additional set of batch experiments (6 days) was carried out to evaluate the adhesion of each strain to steel surfaces under similar conditions to those found in the cooling tower experiments. Substantial differences were observed in the microbial communities that developed in the batch systems and cooling towers. P. aeruginosa showed a low degree of adherence to steel surfaces both in batch and in the cooling towers, but grew much faster than K. pneumoniae and Flavobacterium in mixed-species biofilms and ultimately became the dominant organism in the closed batch systems. However, the low degree of adherence caused P. aeruginosa to be rapidly washed out of the open cooling tower systems, and Flavobacterium became the dominant microorganism in the cooling towers in both the short-term and long-term experiments. These results indicate that adhesion, retention and growth on solid surfaces play important roles in the bacterial community that develops in cooling tower systems.

Figure 1

Schematic of pilot scale cooling tower system.

Figure 2

Surface zeta potential and contact angle of three bacterial strains, P. aeruginosa, K. pneumoniae and Flavobacterium sp., as a function of solution pH. Error bars represent standard deviations (SDs) of 10 replicate measurements.

Figure 3

Concentrations of three introduced bacterial strains in liquid phase (A and C) and on coupon surfaces (B and D) observed in batch studies (A and B) and cooling tower experiments (C and D). Error bars represent SDs of replicate experiments. (–––●––– P. aeruginosa, – –∇– – K. pneumoniae, – – ∎ – – Flavobacterium sp.).

Figure 4

Concentrations of three introduced bacterial strains in liquid phase (A) and on coupon surfaces (B) in cooling tower long-term study. Error bars represent SDs of triplicate experiments. Arrows represent the date when bacteria culture was reinoculated. (–––●––– K. pneumoniae, – – ∇ – – P. aeruginosa, – – ∎ – – Flavobacterium sp., – – ◊ – – total bacteria).

Figure 5

Bacteria concentration in liquid phase (A) and on coupon surfaces (B) observed in mono-species batch experiments. Error bars represent SDs of triplicate experiments. (–––●––– P. aeruginosa, – – ∇ – – K. pneumoniae, – – ∎ – – Flavobacterium sp.).

Figure 6

Confocal images for the surface attached cells on coupons in cooling tower systems. Bar = 50 µm.