UV Resistance of bacteria from the Kenyan Marine cyanobacterium Moorea producens

Abstract

UV resistance of bacteria isolated from the marine cyanobacterium Moorea producens has not been observed previously, findings which highlight how unsafe germicidal UV irradiation for sterilization of air, food, and water could be. Further, UV resistance of Bacillus licheniformis is being observed for the first time. This study focused on bacteria isolated from the marine cyanobacterium M. producens collected off the Kenyan coast at Shimoni, Wasini, Kilifi, and Mida. UV irradiance of isolates (302 nm, 70 W/m² , 0-1 hr) established B. licheniformis as the most UV resistant strain, with the following order of taxon resistance: Bacilli> γ proteobacteria > Actinobacteria. UV resistance was independent of pigmentation. The maximum likelihood phylogenetic distance determined for both B. licheniformis and Bacillus aerius relative to M. producens CCAP 1446/4 was 2.0. Survival of B. licheniformis upon UV irradiance followed first-order kinetics (k = 0.035/min, R² = 0.88). Addition of aqueous extracts (2, 10, 20 and 40 mg/ml) of this B. licheniformis strain on the less resistant Marinobacterium stanieri was not significant, however, the commercial sunscreen benzophenone-3 (BP-3) positive control and the time of irradiance were significant. Detection of bacteria on M. producens filaments stained with acridine orange confirmed its nonaxenic nature. Although the chemistry of UV resistance in cyanobacteria has been studied in depth revealing for example the role of mycosporine like amino acids (MAAs) in UV resistance less is known about how bacteria resist UV irradiation. This is of interest since cyanobacteria live in association with bacteria.

Keywords: Bacillus licheniformis; Moorea producens; UV resistance; bacteria; benzophenone-3; first-order kinetics.

Figure 1

Acridine orange stained bacterial cells on the surface of a live Moorea producens filament

Figure 2

Phylogeny of UV resistant bacteria in relation to Moorea producens CCAP 1446/4

Figure 3

A graph for estimating the population of B. licheniformis following UV irradiance

Figure 4

A survival curve for B. licheniformis under UV radiation. As the time of exposure increases, there is less variation. The survival constant k values are dependent on the total fluence H in the equation $k=(\mathbf{H}\left({\mathbf{e}}^{-\mathbf{t}}-1\right){\text{ln}}_t/{\mathrm{N}}_0$ . H itself is a function of the absolute transmittance T, the bandwidth of radiation b (nm), the fluence H ₀ (W/m²) and time t (min)

Figure 5

A graph showing the effect of concentrations of B. licheniformis AEs on the number of resistant colonies in CFU upon UV irradiance at 15, 30, 45, and 60 min, respectively. The results are the means of three independent experiments. The graph shows that at 60 min, no colonies were observed except for M. stanieri. Colonies were observed at lower times of irradiance below 45 min

Figure 6

Images of marine agar petri dishes spread with Bacillus licheniformis (LHS) and Marinobacterium stanieri (RHS). Both dishes were treated with 100 μl of 20 mg/ml of B. licheniformis metabolite extract after UV irradiation (302 nm, 70 W/m², 45 min) and incubated (32°C, 48 hr). Cells are randomly distributed on the petri dish for B. licheniformis and growing at petri dish edges for M. Stanieri