STRUCTURE OF TYPICAL BACTERIAL CELL
The outer layer or cell envelope consists of two components, one is a rigid cell wall and the second one is beneath it called cytoplasm or plasma membrane. The cell envelope encloses the protoplasm, comprising the cytoplasm, cytoplasm inclusions such as ribosomes and mesosomes, granules vacuoles and the nuclear body. The cell may be enclosed in a loose slime layer called as a capsule. Some bacterial carry filamentous appendages protruding form the cell surface – the flagella which are organs of locomotion and the fimbriae which appear to be organs for attachment.
The Cell Wall
The cell wall accounts for the shape of the bacterial cell and give it rigidity.
Cytoplasmic Membrane
The cytoplasmic (plasma) membrane is a thin layer lining the inner surface of the cell wall and separating it from the cytoplasm.
Cytoplasm
The bacterial cytoplasm is a colloidal system of a variety of organic and inorganic solutes in a viscous watery solution.
Ribosomes
Ribosomes are the centers of protein synthesis.
Mesosomes
Mesosomes are formed as invaginations of the plasma membrane into the cytoplasm. They are to the principal sites of respiratory enzymes in bacteria.
Intracytoplasmic Inclusions
Intracytoplasmic inclusions may be of various types, the chief of which are volutin, polysaccharide, lipid and crystals.2
Nucleus
Bacterial nuclei have no nuclear membrane and nucleolus. It consists of a single molecule of double stranded DNA arranged in the form of a circle.
CLASSIFICATION OF BACTERIA DEPENDING UPON THEIR MORPHOLOGY
Cocci
Round or oval bacteria
Diplococci
Cocci in pairs e.g. Meningococci, Gonococci, Diplococcus pneumoniae
Two round
Staphylococcus
Cocci in clusters
Streptococci
Cocci in chains.
Bacilli
Bacilli are rod or stick shaped bacteria.
Streptobacilli
Bacilli in chains
Diplobacilli
Bacilli in pairs
Vibrios
Slightly curved rods
Spirilla
Regularly coiled rigid organisms
Spirochetes
Flexible, coiled organisms
IMPORTANT PATHOGENS AND THE DISEASES THEY CAUSE
Gram-positive cocci |
S.No | Organism | Morphology | Important disease |
---|---|---|---|
1. | Staphylococcus aureus | Cocci in groups | Boils, abscesses, bacteraemia, pneumonia, food poisoning. |
2. | Streptococcus pyogenes (Group A Streptococcus) | Cocci in chains | Sore throat, bacteraemia and rheumatic fever leading to heart disease. |
3. | Streptococcus pneumoniae (Diplococcus pneumoniae) | Capsulated diplococci | Meningitis pneumonia, otitis |
Gram-positive Rods |
S.No | Organism | Morphology | Important disease |
---|---|---|---|
1. | Bacillus anthracis | Large, spore – forming, capsulated, bacilli which tend to form chains | Anthrax |
2. | Corynebacterium diphtheriae | Pleomorphic nonmotile rods | Diphtheria of the throat and skin |
3. | Listeria monocytogenes | Small rods with tumbling motility at low temperatures | Meningitis, stillbirth, bacteraemia |
4. | Clostridium botulinum | Nonmotile plemorphic rods, with oval subliminal spores. | Severe food-poisoning |
5. | Clostridium tetani Drumstick bacillus | Nonmotile, long thin rods with round terminals spores | Tetanus |
Gram-negative rods |
S.No | Organism | Morphology | Important disease |
---|---|---|---|
1. | Haemophilus influenza | Small, nonmotile cocco-bacilli (or) rods pleomor-phic | Acute respiratory infections, meningitis, ear infections |
2. | Bordetella pertussis | Small, nonmotile cocco-bacilli | Whooping cough |
3. | Brucella species | Small, nonmotile cocco-bacilli | Brucellosis (undulant fever) |
4. | Yersinia pestis (Pasteurella pestis) | Nonmotile, capsulated cocco-bacilli | Septicemia |
5. | Francisella tularensis | Small, nonmotile, capsulated | Skin infections, eye infections, and respiratory infections. |
6. | Bacteroides species | Nonmotile, small pleomorphic rods | Bedsores, abscesses, abdominal infections |
7. | Escherichia coli | Motile (or) nonmotile rods | Urinary infections |
8. | Klebsiella pneumonia | Nonmotile capsulated rods | Chest infections |
9. | Proteus species | Motile rods | Urinary infections |
10. | Pseudomonas aeruginosa (Pseudomonas pyocyanea) | Motile rods | Urinary infections |
11. | Shigella species | Nonmotile rods (or) Coccobacilli | Bacillary dysentery |
12. | Salmonella species | Motile rods | Enteric fever (typhoid paratyphoid) food poisoning, septicemia, meningitis |
13. | Vibrio cholera | Motile-slightly curved rods | Cholera |
14. | Vibrio parahaemolyticus | Motile-slightly curved rods | Gastroenteritis |
15. | Campylobacter jejuni, coli | Motile, curved spiral rods | Gastroenteritis |
16. | Legionella pneumophila | Nonmotile rods | Severe pneumonia |
Gram-negative Cocci |
Neisseria meningitides | Intracellular diplococcic | Meningitis and septicemia |
Neisseria gonorrhoeae | Intracellular diplococcic | Gonorrhoea, eye infection in newborns |
Acid Fast Bacilli |
Mycobacterium tuberculosis | Nonmotile bacilli | Tuberculosis |
Mycobacterium leprae | Nonmotile bacilli after in ten in groups (globi) | Leprosy |
CLASSIFICATION OF THE MEDICALLY IMPORTANT VIRUS GROUPS
RNA VIRUSES |
S.No | Virus group | Shape | Envelope |
---|---|---|---|
1. | Toga virus | Spherical | Yes |
2. | Reovirus | Spherical | No |
3. | Bunya virus | Spherical | Yes |
4. | Arenavirus | Spherical | Yes |
5. | Rhabdovirus | Elongated (Bullet-shaped) | Yes |
6. | Paramyxovirus | Spherical filamentous | Yes |
7. | Picornavirus | Spherical | No |
8. | Orthomyxovirus | Spherical filamentous | Yes |
9. | Coronavirus | Spherical pleomorphic | Yes |
10. | Retrovirus | Spherical | Yes |
DNA VIRUSES |
S.No | Virus group | Shape | Envelope |
---|---|---|---|
1. | Herpesvirus | Spherical | Yes |
2. | Adenovirus | Spherical | No |
3. | Papovavirus | Spherical | No |
UNCLASSIFIED VIRUSES |
S.No | Virus group | Shape | Envelope |
---|---|---|---|
1. | Marburg – Ebola Virus group (RNA) | Filamentous, U – shaped Ring shaped | No |
2. | Hantaan Virus (Probably RNA) | Spherical | No |
3. | Hepatitis A Virus (RNA) | Spherical | No |
4. | Hepatitis B Virus (DNA) | Spherical | No |
VIRUSES
Introduction
Unicellular microorganisms may be classified in the descending order of complexity as the eukaryotes, such as protozoa and fungi and the prokaryotes, such as bacteria, mycoplasma, rickettsiae, and chlamydiae.
Viruses do not fall strictly into the category of unicellular microorganisms, as they do not posses a cellular organization. They contain only one type of nucleic acid, either DNA or RNA but never both.
They are obligates intracellular parasites. They lack the enzyme necessary for proteins and nucleic acid synthesis and are dependent for replication on the synthetic 6machinery of host cells. They multiply by a complex process and not by binary fission. They are unaffected by antibacterial antibodies.
Morphology of Viruses
The extracellular infectious virus practice is called the virion.
Structure and Shape
The virion consists essentially of a nucleic acid and core surrounded by a protein coat, called the capsid. The capsid with the enclosed nucleic acid is known as the nucleocapsid.
The function of the capsid is to protect the nucleic acid from activation by nucleases and other deleterious agents in the environment. The capsid is composed of a large number of capsomere, which form its morphological units.
Chemical capsid is composed of polypeptide molecules, which form a shell around the nucleic acid core. One of the major functions of the capsid is to introduce viral genome into host cell by adsorbing readily to cell surfaces.
Two kinds of symmetry is there within the capsid. One is octahedral (cubical) and other is helical. Virion may be enveloped or nonenveloped (naked) the envelopes or outer covering of viruses is divided from host cell membrane when the progeny virus is released by budding.
The envelope is lipoprotein in nature. Proteins subunits may be seen as projecting spikes on the surface of the envelope called peptonieres.
STRUCTURE OF A TYPICAL FUNGI
MYCOLOGY: Study of Fungus
Introduction
Fungi (singular, fungus) are a group of eukaryotic organisms. Fungi comprise the molds and yeast, fungi are spore bearing protist that lack chlorophyll. They divide asexually, sexually or by both process. The thallus (Plural thalli) or body of a fungus essentially consists of two parts, the mycelium (Plural mycelia) and the spore.
The mycelium is a complex of several filaments called hyphae (singular hypha). New hyphae generally arise from a spore which on germination puts out a germ tube or tubes. These germ tubes elongate and branch to form hyphae. Hyphae are composed of an outer tube like surrounding a cavity, the lumen which is filled by protoplasm. Between the protoplasm, and the wall is plasmalemma, a double layered membrane which surrounds the protoplasm.
Fungus posses rigid cell wall containing chitin, mannan and other polysaccharides. Hyphae are normally divided into cells by cross wall referred to as septa. And hyphae, which do not have septa, are called as nonseptate unlike bacterial colonies, fungal colonies spread readily by the hyphae, producing branching growth.
The rate of growth texture, color and form of the colonies is distinct for each species and these characteristics are therefore important in identification.
Depending on cell morphology, fungi can be divided into 4 classes.
Yeast
Yeasts are unicellular fungi which occur as spherical or oval cells and reproduce by simple budding on culture, they form smooth, creamy colonies the only pathogenic yeast is Cryptococcus neoformans.
Yeast-like Fungi
Grow partly as yeast and partly as elongated cells resembling hyphae. The latter form a pseudomycelium. Candida albicans is a pathogenic yeast-like fungus.
Moulds or Filamentous Fungi
Form true mycelia and reproduce by the formation of different type of spore. Dermatophytes are example of pathogenic moulds.
Dimorphic Fungi
Can occur as filaments or as yeast depending upon the condition of growth. In host tissue culture at 37°C they occur as yeasts, which in the soild and in culture at 22ºC. They appear as mould, most fungi causing systemic infection are dimorphic fungi.
MODE OF TRANSMISSION OF VARIOUS DISEASES
Airborne Infections
By inhaling aerosols (airborne droplets) containing pathogens secreted by an infected person, sneezing, coughing, laughing and spitting bacterial respiratory diseases such as tuberculosis, whooping cough, pneumonia.
Viral. e.g. Measles, influenza virus, adenovirus and rhinovirus.
Water or Foodborne Infections
By ingesting pathogens in water or food contaminated with urine or feces from sick persons or disease carries.
Bacterial diseases, e.g. Cholera, typhoid and bacillary dysentery.
Viral diseases, e.g. Enteroviruses, rotaviruses, HAV.
Sexually Transmitted Diseases
Bacterial: syphilis and gonorrhea.
Transfer of Pathogens from Mother to Fetus During Pregnancy
Bacterial-Syphilis, N. gonorrhoeae, Chlamydiae and group B Streptococcus.
Viral: Cytomegalovirus, rubellavirus, HIV.
Pathogens entering wound, cuts or burns by way of contaminated hands, or unsterile instruments. Infections of the skin and tissues, such as boils and abscesses are transmitted in this way.
FUNGAL INFECTIONS
Superficial Mycosis
Fungus infecting the skin, hair and needs.
Ex: Tinea, Candida, dermatophytes like Trichophyton, Microsporum and epidermophyton.
Deep or Systemic Mycosis
Fungus infecting the subcutaneous tissues and/or internal organs.
Subcutaneous – Ex: Cladosporium, Mad hrella spp. Sporottrix, Candida
Systemic – Ex: - Candida, Cryptococcus, Histoplasma, Blastomyces etc.
Opportunistic Infections
Fatal infections in persons who are otherwise debilitated or immunocompromised. Ex: Candida, Aspergillus, Penicillium, Mucor and Rhizopus, etc.