Microbiology and infection

• Microbiology is the study of organisms that cause infectious diseases, which can be grouped into parasites, fungi, bacteria, viruses, prions, and each group has distinctive characteristics that determine how the organisms interact with their hosts and cause disease.
• Viruses, bacteria, fungi, protozoa, and helminths can be classified based on their cell structure, nucleic acid type, and type of nucleus.
• Macroparasites are larger organisms that can be seen with the naked eye, while microparasites are smaller organisms that can only be seen with a microscope.
• The generation time of an organism is the time it takes for one replication product to mature into another replication product.
• Macroparasites have weak and short-lived immunity, while microparasites have strong and long-lived immunity.
• The cell wall is a multilayered structure mainly composed of peptidoglycan (also known as mucopeptide or murein).
• Athlete’s foot (Trichophyton spp) is a type of fungus.
• Ringworm (Microsporum spp) is a type of fungus.
• Oral candidiasis (candida spp.) is a type of fungus.
• Peptidoglycan is a polymer composed of the hexose sugars N-acetylglucosamine and N-acetylmuramic acid (glycan chain) and amino acids (tetrapeptide chain).
• Taxonomy is the science concerned with systematic classification of organisms, and division of organisms into related groups (taxa) is based on similar characteristics.
• Components of a bacterial cell include the essential components: Cell wall, Capsule, Plasma membrane, Flagella, Ribosome, Pili (Fimbriae), Nucleoid, Plasmid, and Spore.
• Bacteria look like simple organisms but are complex and adapted.
• All microbes except viruses are named according to the binomial Linnean system (genus and species).
• Morphological characteristics of bacteria include nature of the cell wall, staining, shape (cocci, bacilli/rods and spirilla/spirochetes), and spore-forming abilities.
• Spores can survive for long periods of time and can infect wounds contaminated with soil.
• Viruses are intracellular parasites that contain either DNA or RNA genomes comprised of novel genetic material encoding structural and functional proteins needed for self-perpetuation.
• The envelope protein is a lipid bilayer derived from the host cell membrane that contains viral glycoproteins (spikes) that project from the membrane.
• New virus particles are formed by assembly of components synthesised using host cell proteins.
• Spores are highly resistant to heat and can only be killed by autoclaving for 15 minutes at 121 °.
• Spores are highly resistant structures produced by some Gram-positive bacteria such as Bacillus and Clostridium species, and are formed in response to adverse conditions.
• Spores are resistant to boiling and can only be killed by solutions designated as sporicidal, such as bleach and other chlorine-based compounds.
• Spores are resistant to heat, chemicals, and drying, and contain bacterial DNA surrounded by a thick keratin-like coat.
• Many cell functions are required for viral propagation, including machinery for translation of viral mRNAs, enzymes for replication of genome and assembly of new virions, transport pathways to reach the sites of replication, viral assembly, etc.
• Biochemical properties of bacteria include metabolism, production of specific enzymes or toxins, and more recently, DNA sequencing of their genome.
• Examples of bacteria infecting humans include intracellular, miscellaneous/poorly staining, poorly staining, and acid-fast stain bacteria such as Chlamydia, Mycoplasma, Mycobacteria, and Corynebacterium.
• Straight and curved rods are examples of bacteria with different morphologies.
• Bacteria can be classified based on their biochemical and metabolic strategies, reproductive processes, and membrane-bound organelles.
• Fungi, protozoa, and helminths can be classified based on their cell structure, nucleic acid type, and type of nucleus.
• The most serious helminth infections are caused by TAPEWORMS (e.g. Taenia solium) and FLUKES (e.g. Schistosoma mansoni).
• Protozoa are single cell organisms with a life cycle that includes a metabolically active growth stage (trophozoite) and a dormant stage (cyst), and may be free-living or their life cycle may require one or more hosts.
• Viruses used for classification are characterized by the type of nucleic acid (DNA or RNA), the number of strands of nucleic acid (ss/ds) and their physical construction (segmented), the polarity of viral genome (positive or negative strand RNA), symmetry of nucleocapsid, and the presence or absence of a lipid envelope.
• Fungi can grow as single cell (yeasts), branched filaments (hyphae) or both (dimorphic fungi), and their cell wall is composed primarily of chitin, as opposed to bacterial cell walls, which are composed of peptidoglycan.
• Transmission of helminths can be direct through swallowing infective stages or by larvae penetrating the skin, or indirect via intermediate non-human hosts.
• Common fungal diseases include Pityriasis vesicolor, Tinea corporis (Ringworm), Tinea pedis (Athlete's foot), Cryptococcosis, Candidiasis, Aspergillosis, and Pneumocystis pneumonia.
• Helminths are multicellular worms that infest many organs of the body, most commonly the gastrointestinal tract, and have complex life cycles that progress from egg to larva to adult.
• Infections of protozoa are acquired through ingestion of contaminated water or food or via insect vectors, and examples of common diseases caused by protozoa include Malaria (Plasmodium spp.), Toxoplasmosis (Toxoplasma gondii), and Leishmaniasis (Leishmania spp.).
• Neisseria spp. are characterized by the type of nucleic acid (DNA or RNA), the number of strands of nucleic acid (ss/ds) and their physical construction (segmented), the polarity of viral genome (positive or negative strand RNA), symmetry of nucleocapsid, and the presence or absence of a lipid envelope.
• Species causing disease may be acquired from the environment or occur as part of the normal flora.
• The stability of virions outside the host correlates with mode of transmission.