Introduction
This chapter explains the major groups of microorganisms that are important in microbiology and human health. These include bacteria, fungi, viruses, rickettsiae and spirochetes. Each group has its own structure, mode of growth, method of reproduction and pattern of disease. Understanding these helps in diagnosis, treatment and prevention of infections.
Study of Bacteria
Bacteria are unicellular, microscopic organisms that can survive in water, soil, food, air and inside living beings. They grow well on laboratory media and multiply by binary fission. Based on shape, bacteria may be cocci (spherical), bacilli (rod-shaped) or spirilla (helical). They are commonly classified as Gram-positive or Gram-negative using Gram staining.
Structure of Bacterial Cell
- Cell envelope: Made of cell wall and plasma membrane.
- Protoplasm: Contains cytoplasm, ribosomes, mesosomes, granules, vacuoles and nuclear body.
- Capsule or slime layer: Protects the cell and helps attachment.
- Pili/Fimbriae: Help in adhesion and gene transfer.
- Flagella: Organs of locomotion.
- Cytoplasm: Viscous solution with proteins, DNA, plasmids and solutes.
Bacteria can be beneficial or harmful. Many help in digestion, vitamin production and fermentation industries. Probiotic strains like Lactobacillus and Bifidobacterium support gut health. Harmful bacteria may cause pneumonia, meningitis, food poisoning and many other infections.
Antibiotics
Antibiotics kill or stop the growth of bacteria by disrupting essential processes like cell wall synthesis or protein synthesis. They do not work against viruses but may be given during viral illness to prevent secondary bacterial infections.
Study of Fungi
Fungi are eukaryotic microorganisms that may live as saprophytes, parasites or commensals. They may appear as yeasts (oval cells) or moulds (long branching hyphae). They grow on routine laboratory media and reproduce by asexual or sexual methods. Fungi are useful in industries for producing antibiotics, enzymes, organic acids and fermented foods. Some fungi cause human diseases.
Candida albicans
Candida albicans is a budding yeast forming pseudohyphae. It causes candidiasis, affecting skin, nails, mouth, esophagus, vagina, and internal organs. Diagnosis is done by Gram staining, KOH mount and culture. Treatments include nystatin, miconazole for local infections, and amphotericin B or ketoconazole for systemic infections.
Penicillium
Penicillium species are common soil fungi. They appear as white to blue-green colonies on Sabouraud’s agar. They produce conidia in a brush-like arrangement. They cause food spoilage and infections like otomycosis and keratitis. Species like P. notatum and P. chrysogenum produce the antibiotic penicillin.
Dermatophytes
Dermatophytes infect keratinised tissues such as skin, hair and nails, causing dermatophytosis or ringworm. They belong to three genera:
- Trichophyton – affects skin, hair and nails
- Microsporum – affects skin and hair
- Epidermophyton – affects skin and nails
They form septate hyphae and produce macroconidia and microconidia in culture. Treatment includes griseofulvin, miconazole and clotrimazole.
Study of Viruses
Viruses are non-cellular infectious agents much smaller than bacteria (20–300 nm). They contain either DNA or RNA, not both. They cannot grow on artificial media and must multiply inside living host cells, so they are called obligate intracellular parasites. Viral diseases include influenza, AIDS, hepatitis, polio and measles.
Structure of Viruses
- Nucleic acid: DNA or RNA, single or double-stranded.
- Capsid: Protein coat made of capsomeres.
- Envelope: Present in some viruses; may contain spikes for attachment.
- Virion: Complete infectious particle.
Types of Viruses
- Bacteriophages: Infect bacteria.
- Plant viruses: Infect plants like potato, tobacco and sugarcane.
- Animal viruses: Infect humans and animals.
Study of Rickettsiae
Rickettsiae are small Gram-negative bacteria showing properties of both bacteria and viruses. They are obligate intracellular parasites transmitted mainly by arthropods such as ticks, lice, mites and fleas. They multiply by binary fission but require living cells for growth.
Characteristics
- Cultivated in chick embryos or cell cultures
- Cause diseases like typhus, scrub typhus, Q-fever and Rocky Mountain spotted fever
- Grow inside endothelial cells of blood vessels
- Possess toxic substances (endotoxins and hemolysins)
They reach humans through bites or contaminated faeces of infected vectors. After entry, they spread in the bloodstream and damage the lining of small blood vessels, leading to necrosis and organ injury.
Study of Spirochetes
Spirochetes are thin, flexible, spiral-shaped bacteria. They are Gram-negative and move with the help of axial filaments. They may be free-living or parasitic. Medically important genera include Treponema, Leptospira and Borrelia.
Treponema pallidum
Treponema pallidum causes syphilis. It is thin, motile and difficult to stain. It is cultivated in rabbit testis because it cannot grow on artificial media. Syphilis progresses through primary, secondary, latent and tertiary stages.
Leptospira
Leptospira interrogans causes leptospirosis, a zoonotic disease transmitted from animals such as rodents, pigs and cattle. It enters the human body through cuts, abrasions or mucous membranes after contact with contaminated water. Symptoms include fever, muscle pain, jaundice and kidney involvement. Weil’s disease is a severe form.
Key Points
- Bacteria, fungi, viruses, rickettsiae and spirochetes are major microbial groups of medical importance.
- Bacteria are prokaryotic, fungi are eukaryotic and viruses are acellular.
- Rickettsiae and spirochetes are specialized microorganisms with unique structural and disease-causing properties.
- Understanding their characteristics helps in accurate diagnosis and appropriate treatment.
Detailed Notes:
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