Introductory Chapter: Highlighting Pros and Cons of Bacterial Biofilms

1. Introduction

Bacteria are unicellular microorganisms that belong to the classification of prokaryotes (other prokaryotic organisms are Archaea) that lack a membrane-bound nucleus (genetic material DNA is present in the cytoplasm) and other organelles such as mitochondria [1, 2]. Bacteria are generally classified in three shapes: rod, sphere/cocci and spiral. They can divide/multiply/grow and metabolise either in the presence of oxygen (aerobic) or in the absence of oxygen (anaerobic). In addition, they can be facultative and survive under aerobic, anoxic (low oxygen) or anaerobic conditions. Bacteria get energy by making adenosine triphosphate (ATP) through glycolysis, pyruvate oxidation, citric acid cycle and the electron transport chain in the presence of oxygen (aerobic respiration), whereas in the absence of oxygen, ATP is produced via fermentation of glycolysis derived products (anaerobic respiration) [1, 3, 4]. In microbiology, bacteria are differentiated into two groups: (i) Gram-positive and (ii) Gram-negative depending upon bacterial ability to retain Gram stain or crystal violet stain. Gram-positive bacteria have thick peptidoglycan cell walls that strongly bind and retain crystal violet stain. In contrast, Gram-negative bacteria have a thinner peptidoglycan cell wall that cannot retain crystal violet stain and hence is washed easily when washed with ethanol. Gram-positive bacteria appear purple or blue after staining, whereas Gram-negative bacteria appear pink when observed under a microscope [1].

2. Introducing bacterial biofilms

Bacteria exist in abundance in almost all corners of the Earth, including marine and freshwater, rocks and soil, in man-made/engineered surfaces such as ships, pipelines and living organisms, including humans, animals, birds and plants. Bacteria are either free-living/planktonic or exist in communities embedded in their self-produced extracellular matrix called “Biofilm”. It has been projected that on Earth, up to 80% of bacterial cells live the biofilm mode of lifestyle [5]. The biofilm stage is the preferred stage in bacterial lifestyles, principally for species with a pathogenic nature, as the biofilm stage provides resistance against physical, chemical and environmental challenges [6]. Biofilm formation is a complex process with multiple steps starting with the initial adhesion of planktonic bacteria to the surface, aggregation, micro-colony formation and proliferation into the mature biofilm and finally, active disruption of biofilms to release planktonic bacterial cells to progress adhesion at new sites [7]. The biofilm formation process involves various biomolecular pathways; the most prominent one is the cell-to-cell signalling pathway in bacteria and is commonly acknowledged as the Quorum Sensing (QS) system [8]. The QS system in bacteria activates in response to the fluctuations in the bacterial population. As the bacterial cell density increases, bacteria produce chemical signals called “autoinducers” that are recognised by the local population to facilitate communication between their own and different bacterial species [8]. The QS system regulates genes essential for the biosynthesis of various products by bacteria, including biopolymers (polysaccharides, DNA and protein—that are essential for biofilm matrix formation and integrity), virulence factors, biofilm formation and protection against physical (hydrodynamic shear stress), chemical, host immune response and antimicrobial challenges [9, 10, 11]. The role of bacteria and its biofilm stage can be beneficial or devastating. Both biofilm applications for beneficial use and biofilm eradication to protect the environment and the health of patients account for a multi-billion-dollar industry annually. Below are the highlights of the pro and cons of bacteria/biofilms.

3. Application of beneficial bacteria in ecosystem and industry

In terms of beneficial bacteria, their applications in the environment and industry are diverse, including maintaining biological balance in natural aquatic and soil ecosystems by remineralisation and restoring nutrients [12]. Rhodococcus spp. of bacterial biofilm has significant application in bioremediation, including cleaning industrial and domestic pollutants in the environment by decaying organic pollutants such as polycyclic aromatic hydrocarbons (e.g. petroleum products) and chlorinated organic compounds from soil and water bodies [13]. Soil bacteria (e.g. Bacillus subtilis, Pseudomonas putida and Rhizobium spp.) that maintain a symbiotic relationship with the plant also promote plant growth by fixing nitrogen in the plant roots, which then converts nitrogen into ammonia essential for plant fitness and development [14]. The use of bacterial secreted by-products in the food and pharmaceuticals industry for commercial use has existed for many decades, such as lipase (e.g. phospholipase) enzyme in making bread (baking) and winemaking brewing) industries, vegetable oil refinement, in the dairy industry to hydrolysis milk fat for cheese production and biodegradation of petroleum products [15]. Microbial amylase is another industrial application enzyme mainly used in the hydrolysing of complex carbohydrates (e.g. starch saccharification) into smaller sugar (glucose and fructose) units in the manufacture of corn syrups [16].

Bacterial biofilms have a more extensive application in biomining, such as the recovery of copper metal and the generation of biogas/coal gas. Some bacterial species, Leptospirillum ferriphilum, Sulfobacillus thermosulfidooxidans and Acidithiobacillus, are used to recover copper from chalcopyrite (CuFeS2); these bacteria catalyse the transformation of solid metal sulfide dissolution to soluble metal sulfates [17]. Methanobacteria is used to produce biogas (methane, carbon dioxide and hydrogen) from organic waste, including cattle and human waste. Biogas’s predominant application is used for cooking and water heating in rural India and is also used in the production of electricity [18, 19].

4. The catastrophic impact of biofilms on the health care sector and the environment

Bacterial biofilms cause catastrophic impacts in terms of infection, antimicrobial resistance and associated morbidity and mortality. Statistics show that more than 80% of chronic infections are associated with biofilm-forming microbes [20]. Some of the common infections associated with bacterial biofilms include urinary tract infection, wound infection, infection in diabetic leg ulcers, medical implant-associated infections including surgical site infection and catheter-associated infections, microbial keratitis mainly in people wearing contact lenses, chronic sinusitis, bacterial pneumonia in chronic obstructive pulmonary patients, cystic fibrosis, HIV patients, COVID-19 patients, infective endocarditis, stomach ulcers, tooth decay and periodontitis infection etc. The burden of biofilm-associated infections is responsible for a global economic loss of hundreds and thousands of billions annually [21]. Some of the common bacterial species that are responsible for the above-mentioned health-associated infections include P. aeruginosa, Acinetobacter baumannii, Klebsiella pneumoniae, Protease mirabilis, and Escherichia coli, Helicobacter pylori, Porphyromonas gingivalis, Staphylococcus aureus, Streptococcus pyogenes, S. pneumoniae and others. In addition, to being directly detrimental to human health, biofilms are also accountable for an economic loss in agriculture, dairy, livestock and the meat industry. Statistical analysis reveals that biofilm infections in plants (fruits and vegetables) account for up to 10% of the world’s food supply loss and are unswervingly responsible for foodborne illnesses [22]. Similarly, a bacterial (e.g., Streptococcus agalactiae) infection in cows (bovine mastitis—inflammation of the mammary glands) contributes to an 11% decrease in US total milk production alongside a two billion dollars monetary loss to the US dairy industry [22].

Biofilm-associated corrosion is an enormous problem in multiple sectors, including the marine and shipping industry (damages to the ships) and chemical processing and water treatment industries (water pipelines, heat exchangers and stainless steel tanks). These bacteria can withstand a wide range of pH 4−9 and temperatures 10−50°C [23]. For example, sulphate-reducing bacteria (grow in anoxic conditions) are a prime culprit in the marine industry corrosion; these bacteria influence changes in the physicochemical parameters such as pH of the local environment and redox potential of the metal [24]. It reduces sulfate to metal sulfide, and the production of hydrogen sulfide gas triggers metal corrosion [25]. Microbial-induced corrosion attributes to a negative impact on the man-made infrastructure and loss of billions of dollars annually [25].

This book “Bacterial Biofilms” collected the chapters written by prominent and expert scientists from their respective areas of research and highlighted the pros and cons of bacterial biofilms in different sectors. The content in this book will educate people from different backgrounds, including but not limited to scientists, doctors, infectious diseases specialities, high school and university students and the public.