BACTERIAL MORPHOLOGY

·       Bacteria can in part be identified using microscopic observations of morphological properties such as:

o     Cell size

o     Cell shape

o     Arrangement with cells of the same species

o     Staining characteristics

·       Bacteria are normally observed microscopically using:

o     Wet preparations observed under phase contrast – bacteria are living and shape, motility, spores and intracellular granules can be observed

o     Heat fixed & stained preparations – bacteria are darkly stained against a bright background

Cell size, shape and arrangement

3 major categories:

Cocci (s. coccus) – spherical

Bacilli (s. bacillus) – rod shaped

Spirals

+

Filamentous – thread like

Pleomorphic – bacteria that change shape

Cocci

·       Spherical

·       0.4 – 2 µm

·       Often arranged in groups as a result of incomplete separation of cells during division:

o     Diplococcus (pair) e.g. Neisseria gonorrhoea

o     Streptococcus (strep = thread) e.g. Streptococcus pyogenes – causes scarlet fever, sore throat, septicaemia etc.

o     Staphylococcus (staphyle = Gk bunch of grapes) – members of this genus form part of skin flora but also cause abscesses, boils, & toxic shock syndrome

Bacilli

·       Rod shaped (usually single)

·       1-10 µm

·       Special forms:

o     Coccobacilli – short stumpy rods – appear ovoid

o     Vibrios – Comma shaped e.g. Vibrio cholerae – causes the gastrointestinal disease cholera

o     Palisade – organisms bend at point of division – form a palisade arrangement e.g. Corynebacterium diphtheriae – causative agent of diphtheria

Spirals

·       Best observed with dark field / phase contrast microscopy

·       Occur singly

·       2 groups:

o     Spirilla (rigid spirals) e.g. Spirillum minor – causes rat bite fever

o     Spirochaetes (flexible spirals) e.g. Treponema pallidum – causes syphilis

Filamentous

·       Long strands composed of many cells

·       Filaments resemble fungal hyphae but are half the diameter

·       E.g. Actinomycetes

Pleomorphic

·       Bacteria that have a changing and undefined shape 

·       Are the smallest prokaryotic organisms (200-250nm diameter)

·       E.g. mycoplasmas – pathogens causing ‘nonbacterial’ pneumonia and non-gonococcal urethritis

The flagellum as an identification feature

·       Cocci never possess flagella

·       Flagellate rods show a variety of arrangements:

o     Polar – flagella are on the ends of cells e.g. Monotrichous (single flagellum)

Lophotrichous (many flagella in a tuft at one end)

Amphitrichous (single or tufts of flagella at both ends)

o     Peritrichous – flagella cover the cell surface

o     Lateral – flagella at the side

Staining characteristics of bacteria

Several types of stains can be used:

·       Simple stains

·       Differential stains

·       Structural stains

Simple stains

·       Basic stains e.g. methylene blue, methyl violet, basic fuchsin are used for bacteria

·       These colour acidic cell components i.e. basophilic structures e.g. nucleic acid (cf acid stains colour acidophilic structures e.g.cytoplasm)

Differential stains E.g,

·       Gram stain

·       Ziehl Neelsen method i.e. acid fast staining reaction used for certain bacteria that have a high lipid content and resist simple staining

The bacteria are stained with hot basic fuchsin in aqueous 5% phenol and resist decolourization with20% sulphuric acid

Mycobacteria causing tuberculosis and leprosy are identified by this method

Structural stains

·       Endospore stains e.g. malachite green / safranin

·       Capsule stain e.g. Congo red

·       Flagella stain e.g. Leifson’s method