Bacillus (Latin "stick") is agenus ofGram-positive, rod-shapedbacteria, a member of the phylumBacillota, with 266 namedspecies. The term is also used to describethe shape (rod) of other so-shaped bacteria; and the pluralBacilli is the name of theclass of bacteria to which this genus belongs.Bacillus species can be eitherobligate aerobes which are dependent onoxygen, orfacultative anaerobes which can survive in the absence of oxygen. CulturedBacillus species test positive for theenzymecatalase if oxygen has been used or is present.[1]
Bacillus can reduce themselves to ovalendospores and can remain in this dormant state for years. The endospore of one species from Morocco is reported to have survived being heated to 420 °C.[2] Endospore formation is usually triggered by a lack of nutrients: the bacterium divides within its cell wall, and one side then engulfs the other. They are not truespores (i.e., not an offspring).[3] Endospore formation originally defined the genus, but not all such species are closely related, and many species have been moved to other genera of theBacillota.[4] Only one endospore is formed per cell. The spores are resistant to heat, cold, radiation, desiccation, and disinfectants.Bacillus anthracis needs oxygen to sporulate; this constraint has important consequences for epidemiology and control.[5] In vivo,B. anthracis produces a polypeptide (polyglutamic acid) capsule that kills it from phagocytosis.[5] The generaBacillus andClostridium constitute the familyBacillaceae. Species are identified by using morphologic and biochemical criteria.[1] Because the spores of manyBacillus species are resistant to heat, radiation, disinfectants, and desiccation, they are difficult to eliminate from medical and pharmaceutical materials and are a frequent cause of contamination. Not only are they resistant to heat, radiation, etc., but they are also resistant to chemicals such as antibiotics.[6] This resistance allows them to survive for many years and especially in a controlled environment.[6]Bacillus species are well known in the food industries as troublesome spoilage organisms.[1]
Many species ofBacillus can produce copious amounts of enzymes, which are used in various industries, such as in the production ofalpha amylase used in starch hydrolysis and theproteasesubtilisin used indetergents.B. subtilis is a valuable model for bacterial research. SomeBacillus species can synthesize and secretelipopeptides, in particularsurfactins andmycosubtilins.[9][10][11]Bacillus species are also found in marinesponges.[11] Marine sponge associatedBacillus subtilis (strains WS1A and YBS29) can synthesize several antimicrobial peptides.[11][12] TheseBacillus subtilis strains can develop disease resistance inLabeo rohita.[11]
The cell wall ofBacillus is a structure on the outside of the cell that forms the second barrier between the bacterium and the environment, and at the same time maintains the rod shape and withstands the pressure generated by the cell'sturgor. The cell wall is made ofteichoic and teichuronic acids.B. subtilis is the first bacterium for which the role of anactin-likecytoskeleton in cell shape determination andpeptidoglycan synthesis was identified and for which the entire set of peptidoglycan-synthesizing enzymes was localized. The role of the cytoskeleton in shape generation and maintenance is important.[13]
Bacillus species are rod-shaped, endospore-forming aerobic or facultatively anaerobic, Gram-positive bacteria; in some species cultures may turn Gram-negative with age. The many species of the genus exhibit a wide range of physiologic abilities that allow them to live in every natural environment. Only one endospore is formed per cell. The spores are resistant to heat, cold, radiation, desiccation, and disinfectants.[1]
The genusBacillus was named in 1835 byChristian Gottfried Ehrenberg, to contain rod-shaped (bacillus) bacteria. He had seven years earlier named the genusBacterium.Bacillus was later amended byFerdinand Cohn to further describe them as spore-forming, Gram-positive, aerobic or facultatively anaerobic bacteria.[14] Like other genera associated with the early history of microbiology, such asPseudomonas andVibrio, the 266 species ofBacillus are ubiquitous.[15] The genus has a very largeribosomal16S diversity.[16]
Established methods for isolatingBacillus species for culture primarily involve suspension of sampled soil in distilled water, heat shock to kill off vegetative cells leaving primarily viable spores in the sample, and culturing on agar plates with further tests to confirm the identity of the cultured colonies.[17] Additionally, colonies which exhibit characteristics typical ofBacillus bacteria can be selected from a culture of an environmental sample which has been significantly diluted following heat shock or hot air drying to select potentialBacillus bacteria for testing.[18]
Cultured colonies are usually large, spreading, and irregularly shaped. Under the microscope, theBacillus cells appear as rods, and a substantial portion of the cells usually contain ovalendospores at one end, making them bulge.[19]
S.I. Paul et al. (2021)[11] isolated and identified multiple strains ofBacillus subtilis (strains WS1A,[20] YBS29,[21] KSP163A,[22] OA122,[23] ISP161A,[24] OI6,[25] WS11,[26] KSP151E,[27] and S8,[28]) frommarine sponges of theSaint Martin's Island Area of theBay of Bengal,Bangladesh. Based on their study, colony, morphological, physiological, and biochemical characteristics ofBacillus spp. are shown in the Table below.[11]
Ash and Carol (2008) also uses 16S rRNA and found extensive "phylogenetic heterogenity".[29]
'The All-Species Living Tree' Project, which has been in operation since 2008, also maintains a 16S (and 23S if available) tree of all validated species.[31][32][33] In this tree, the genusBacillus contains a very large number of nested taxa and majorly in both 16S and 23S. It isparaphyletic to theLactobacillales (Lactobacillus, Streptococcus, Staphylococcus, Listeria, etc.), due toBacillus coahuilensis and others.[34]
Alcarazet al. 2010 presents a geneconcatenation study, which found results similar to the All-Species Living Tree, but with a much more limited number of species in terms of groups.[35] (This scheme usedListeria as an outgroup, so in light of the ARB tree, it may be "inside-out").
Nikolaidiset al. 2022 studied 1104Bacillus proteomes using a gene concatenation based on 114 core proteins and delineated the relationships among the various species, defined asBacillus from the NCBI taxonomy.[38] The various strains were clustered into species, based on Average Nucleotide identity (ANI) values, with a species cutoff of 95%.[38]
One clade, formed byBacillus anthracis,Bacillus cereus,Bacillus mycoides,Bacillus pseudomycoides,Bacillus thuringiensis, andBacillus weihenstephanensis under the 2011 classification standards, should be a single species (within 97% 16S identity), but for medical reasons, they are considered separate species[39] (an issue also present for four species ofShigella andEscherichia coli).[40]
Bacillus subtilis (natto) is the key microbial participant in the ongoing production of the soya-based traditional natto fermentation, and someBacillus species are on the Food and Drug Administration's GRAS (generally regarded as safe) list.[54]
The capacity of selectedBacillus strains to produce and secrete large quantities (20–25 g/L) of extracellular enzymes has placed them among the most important industrial enzyme producers.[citation needed] The ability of different species to ferment in the acid, neutral, and alkaline pH ranges, combined with the presence of thermophiles in the genus, has led to the development of a variety of new commercial enzyme products with the desired temperature, pH activity, and stability properties to address a variety of specific applications. Classical mutation and (or) selection techniques, together with advanced cloning and protein engineering strategies, have been exploited to develop these products.[citation needed]
Efforts to produce and secrete high yields of foreign recombinant proteins inBacillus hosts initially appeared to be hampered by the degradation of the products by the host proteases.[citation needed] Recent studies have revealed that the slow folding of heterologous proteins at the membrane-cell wall interface of Gram-positive bacteria renders them vulnerable to attack by wall-associated proteases.[citation needed] In addition, the presence of thiol-disulphide oxidoreductases inB. subtilis may be beneficial in the secretion of disulphide-bond-containing proteins. Such developments from our understanding of the complex protein translocation machinery of Gram-positive bacteria should allow the resolution of current secretion challenges and makeBacillus species preeminent hosts for heterologous protein production.[citation needed]
Bacillus strains have also been developed and engineered as industrial producers of nucleotides, the vitamin riboflavin, the flavor agent ribose, and the supplement poly-gamma-glutamic acid. With the recent characterization of the genome ofB. subtilis 168 and of some related strains,Bacillus species are poised to become the preferred hosts for the production of many new and improved products as we move through the genomic and proteomic era.[55]
Bacillus subtilis is one of the best understood prokaryotes, in terms of molecular and cellular biology. Its superb genetic amenability and relatively large size have provided the powerful tools required to investigate a bacterium from all possible aspects. Recent improvements influorescent microscopy techniques have provided novel insight into the dynamic structure of a single cell organism. Research onB. subtilis has been at the forefront of bacterial molecular biology and cytology, and the organism is a model for differentiation, gene/protein regulation, and cell cycle events in bacteria.[56]
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