Biofilm Formation Capability of Cowpea Nodular Bacterial Isolates in Static Microcosms

Abstract

Bacteria are known to form microbially derived sessile communities that are irreversibly attached to biotic or abiotic surfaces known as biofilms. Biofilm formation is an adaptive strategy for the successful colonization of plant surfaces. Forty bacterial isolates of cowpea nodules from 12 smallholder farms in Northern Nigeria were evaluated for biofilm growth, attachment and strength in static microcosms. Replicate King’s B (KB) microcosms (n = 8) were incubated statically for 7 days before assessments. Growth and attachment were determined by measuring optical density (OD600nm) and absorbance (A570 nm) respectively. Strength (grams) was determined by maximum deformation mass (MDM) assay using glass balls (mean weight of 0.0115g). A significant MDM was recorded when more than one replicate retained at least one ball for at least 5 seconds. One-way Analysis of Variance (ANOVA) was used to determine significant mean differences (p<0.05). Similarities and variations in biofilm formation were assessed by Hierarchical Cluster Analysis (HCA) using the Ward method. Half of the isolates showed good biofilm growth and attachment. Twelve of the 40 isolates showed various levels of strength. Isolates 16F and 39L were rated well in all three biofilm parameters. There were significant differences in the mean values of the assessed biofilm parameters among the isolates. Mean growth ranged between 1.493 (Isolate 32K) and 2.101 OD600 (Isolate 19G). Mean attachment levels (A570) ranged from 0.084 (Isolate 9D) to 1.543 (Isolate 30K). Only isolate 2A had significantly higher biofilm strength (2.412g) than the rest of the isolates. HCA analysis indicated similarities as well as variations in biofilm formation by isolates within and between sites. Isolates from all the study sites were positive for at least one of the tested biofilm parameters. The ability of the isolates to form biofilms indicated that some of them could be suitable for biotechnological applications such as in biofertilizer production