Spontaneous generation is the belief that living organisms could arise from non-living things. Biogenesis is the theory that all living organisms come from living things. Bacteria are small in size and naked to the human eye. They are made from prokaryotic cells. Prokaryotic cells are unicellular organisms without a nucleus or membrane-enclosed organelles. These cells differ from eukaryotic cells which are found in plants and animals with a nucleus and membrane-enclosed organelles.
Bacteria are made up of the following components: cytoplasm, ribosomes, nucleoid, plasma membrane, complex and rigid cell wall, capsule, flagella, pili/fimbriae, and endospores. The cytoplasm consists of cytosol which is a jelly-like aqueous solution. The cytoplasm consists of three main groups of molecules: macromolecules — proteins, mRNA, and tRNA; small molecules — energy sources, precursors of macromolecules, metabolites, or vitamins; and inorganic ions — co-factors required for enzymatic activity. The function of the cytoplasm is to facilitate chemical reactions and dissolve solutes. The cytoplasm in bacteria contains the nucleoid (DNA) and ribosomes for protein synthesis. The nucleoid contains the bacterial genetic material. The bacterial genome is a single haploid circular chromosome containing double-stranded DNA. Proteins are involved in DNA compaction and are transcription factors for the regulation of genome expression. RNA is divided into mRNA which is used for coding proteins and ncRNA which is involved in DNA organization and expression of the bacterial genome. The plasmid is a circular molecule of DNA that replicates separately from the genome. The plasmid’s function is to contain the genes associated with disease (virulence factors) or to survive in presence of antibiotics and other toxic compounds (resistance genes). There also exists conjugative plasmids that allow the cell-to-cell transfer by conjugation. Ribosomes are ribonucleoproteins with a large 50S subunit and small 30S subunit. They differ from the eukaryotic cell ribosomes. Ribosomes are the target for antibiotics. The bacterial envelope contains a cytoplasmic or plasma membrane, cell wall, periplasmic space, outer membrane (Gram-negative only), and capsule (only in some bacteria). The plasma membrane is the innermost layer and is composed of a phospholipid bilayer and proteins. Only hydrophobic molecules can enter the cell by passive diffusion through the bilayer. The proteins in the membrane are transport proteins or energy generation components used to synthesize ATP. The plasma membrane’s functions are to be a selective permeability barrier, bacterial respiration and energy generation, and anchor for external structures. The cell wall is made up of a layer of peptidoglycan, a polymer in prokaryotic cells. The structure consists of chains that alternate from the subunits of N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM) cross-linked by transpeptidase enzymes or penicillin-binding proteins (PBPs). The function of the cell wall is for protection and transport. The outer membrane of Gram-negative bacteria is a protein-containing asymmetrical lipid bilayer. The lipopolysaccharide or endotoxin makes up this layer and provides a virulence factor. The function of the outer membrane is to be a selective permeability barrier, resistant to large or hydrophobic toxic compounds, and have a tolerance to detergents and bile salts. There are two exceptions to the two main structural classes of bacteria: mycobacteria and mollicutes (mycoplasma). The mycobacteria are Gram-positive bacteria that contain mycolic acid. The mollicutes have a plasma membrane with sterols and no cell wall. The capsule is made up of glycocalyx and is a polysaccharide layer. The function of the capsule is for protection, a nutrient reserve, and serves as a virulence factor. The endospores are a durable cell type. The function of the endospore is to ensure survival during harsh environmental conditions. The flagella functions as locomotion for bacterial motility. The pili/fimbriae are fine straight hairs on the cell wall that function as adhesion to host tissues and contribute to antigenicity.
The bacterial cell can come in a few shapes and sizes. The most common are the cocci, bacilli, budding, and appendaged bacteria. The growth of a single bacterium can produce a bacterial colony that is visible without a microscope on growth media.
There a few factors required for bacteria to grow. Bacteria replicate via binary fission. The generation time determines how long of time is required for a single bacterial cell to yield two daughter cells. The bacterial growth curve has 4 phases. The lag phase allows the cell to increase in size and have an active metabolism. No division is occurring. The exponential phase/logarithmic phase is where cells multiply at a maximum rate. The maximal stationary phase happens when the exhaustion of nutrients or accumulation of toxic products occurs, the growth will cease. There is a balance between slow multiplication and death rate. The decline phase or death phase is the progressive death of cells.
Fungi have important characteristics. These creatures are eukaryotic, the cell membrane contains sterols (ergosterol) and a plant-like cell wall with chitin, glucan, and mannoproteins. They also have branching hyphae multicellular molds such as mycelium. They can also be unicellular yeasts that spread by budding.
Bacterial pathogenesis describes the biological mechanisms that lead to disease. The pathogenicity of an organism describes its ability to cause disease. The diseases can be carried by different carriers: convalescent — those who recovered but continue to shed the pathogen, healthy (subclinical) — carrier state without clinical symptoms but shedding the pathogen, and incubatory — a carrier that is incubating the pathogen but not yet ill. Strangles in horses detects convalescent carriers.
Bacterial virulence is the measurement of the pathogenicity of the bacteria. Virulence factors are the characteristics that contribute to the virulence whether physical or chemical components. Pathogenicity islands carry gene coding for one or more virulence factors. Horizontal gene transfer of bacteria can happen through three means: bacterial transformation, bacterial transduction, or bacterial conjugation. Quorum sensing is the regulation in gene expression in response to fluctuations in cell-population density, mediated by chemical signal molecules (autoinducers).
Adherence and colonization have the help of a few factors: flagella, pili/fimbriae, and capsule made of glycocalyx.
Invasion is the ability of the pathogen to spread to other locations in the host, by invading host cells or tissue. Extracellular invasion is when bacteria break down barriers of tissue to disseminate in the host while remaining outside of host cells. Intracellular invasion is when the bacteria penetrate cells and survive within the environment.
Exotoxins are generally proteinaceous toxins. They deliver by either secretion or by direct injection.
Endotoxins are toxic components of the prokaryotic cell wall that are not released until cell death and lysis of the bacteria.
Biofilms are masses of bacteria that cling to surfaces, producing an extracellular polymer matrix and exchange nutrients. Their functions are bacterial persistence, reduction of host immunity, local damage, and reduced susceptibility to antibiotics.
Iron uptake: Vertebrate tissue free of iron and iron is needed for bacterial growth. Bacterial cytotoxins damage host cells to release ferritin, hemoglobin, and lactoferrin. Receptor-mediated recognition also allows iron to be taken from the “battlefield”.
