Short communication: A competitive exclusion study reveals the emergence of Bacillus subtilis as a predominant constitutive microorganism of a whey reverse osmosis membrane biofilm matrix

Abstract

Microbial attachment and colonization on separation membranes lead to biofilm formation. Some isolates within the biofilm microflora acquire greater resistance to the chemical cleaning protocols on prolonged use of membranes. It is thus likely that the constitutive microflora might compete with each other and result in certain species emerging as predominant, especially within older biofilms. To understand the microbial interactions within biofilms, the emergence of predominance was studied in the current investigation. An 18-mo-old reverse osmosis membrane was procured from a whey processing plant. The membrane pieces (2.54 × 2.54 cm2) were neutralized by dipping in Letheen broth. The resuscitation step was done in tryptic soy broth (TSB) at 37°C, followed by plating on tryptic soy agar (TSA) to recover the constitutive microflora. Distinct colonies of isolates were further identified using MALDI-TOF as Bacillus licheniformis, Exiguobacterium aurantiacum, Acinetobacter radioresistens, Bacillus subtilis (rpoB sequencing), and 1 unidentified species each of Exiguobacterium and Bacillus. Further, the competitive exclusion study helped establish the emergence of predominance using a co-culturing technique. Fifteen combinations (of 2 isolates each) were prepared from the isolates. Pure cultures of the respective isolates were spiked in a ratio of 1:1 in TSB and incubated at 37°C for 24 h, followed by plating on TSA. The enumerated colonies were distinguished based on colony morphology, Gram staining, and MALDI-TOF to identify the type of the isolate. Plate counts of B. subtilis emerged as predominant with mean log counts of 7.22 ± 0.22 cfu/mL. The predominance of B. subtilis was also validated using the process of natural selection in a multispecies growth environment. In this instance, the TSB culture with overnight-incubated membrane piece (with mixed-species biofilm) at 37°C for 12 h was inoculated in fresh TSB and incubated for the second cycle. Overall, 5 such sequential broth-culture incubation cycles were carried out, followed by pour plating on TSA plates, at the end of each cycle. The isolates obtained were identified using a similar methodology as mentioned above. The fifth subsequent transfer depicted the presence of only 1 B. subtilis isolate on plating, thereby validating its predominance under the conditions of the experiment.