Selected bacterial species classification and their characteristics

                             1. Gram‐Positive Rod-shaped Bacteria.

•The genera of importance in this group are:-Bacillus, Clostridium and Corynebacterium.

1.1: Bacillus :

Occurrence: Members of this group are widespread in air, soil and water and in animal products such as hair, wool and carcasses.

Characteristics: occurs as large rod with square ends; it is aerobic and spore-forming.

Members: The most dangerous member of the group,

1.1.1: Bacillus anthracis, is the causative organism of anthrax.

1.1.2: B.cereus is the cause of food poisoning,

1.1.3: B.polymyxa is the source of the antibiotic polymyxin,

1.1.4: B. brevis is the source of tyrothricin.

1.1.5: B.subtilis and B.licheniformis is the source of bacitracin antibiotic.

1.2: Clostridium:

Characteristics: Clostridia are anaerobic, spore-forming rods.

Members: The genus contains a number of dangerous pathogens:

1.2.1:Clostridium septicum, Cl. perfringens (welchii) and Cl. novyi (oedematiens) cause serious damage to tissue if able to develop in wounds where the oxygen supply is limited.

•Tissue may be destroyed and carbon dioxide produced from muscle glycogen gives rise to the condition known as gas gangrene.

•i.e. they are causative organisms of gas gangrene.

1.2: Clostridium members cont’d:

1.2.2: Cl.botulinum: secretes an extremely toxic nerve poison and ingestion of food in which this organism has grown is fatal.

• Cooking rapidly destroys the poison but cold meats and sausages which contain the organism and which are eaten uncooked, are possible source of botulism.

1.2.3: Cl.tetani: also produces a powerful central nervous system poison and give rise to the condition known as lockjaw or tetanus.

1.3. Corynebacterium:

•are Gram-positive rods found in various environments, including the soil.

Members:

1.3.1: Corynebacterium diphtheriae:  is non-spore forming rods. It is a causative organism of diphtheria, a disease which has largely been eradicated by immunisation.

1.3.2: Gardnerella vaginalis: (previously named Corynebacterium vaginale or Haemophilus vaginalis).

•Although is often part of the normal flora of the vagina, can be a cause of vaginitis.

•It has been suggested that vaginalis is expressed in association with anaerobes.

•It responds to treatment with metronidazole.

1.4: Actinomyces :

•Actinomyces species are Gram-positive rods that assume many shapes and usually form branching filaments.

•Most species are anaerobic, and one species is responsible for a human and cattle infection called lumpy jaw. 

1.5: Lactobacilli.

Characteristics: Lactobacilli are Gram-positive, rod-shaped, non-sporing, bacteria occurring as single cells or chains.

•They produce lactic acid in their metabolism and are associated with the flora of the mouth, vagina and animal faeces.

•Certain species are associated with the production of dairy products such as yogurt, sour cream, and buttermilk.

2. Gram‐Negative Rod-shaped Bacteria

2.1: Pseudomonas:

Characteristics: are aerobic, Gram-negative rods that are motile with polar flagella. Over 30 species are found in the group.

Members:

 2.1.1: Pseudomonas aeruginosa,(pyocyanea), dangerous pathogen that causes urinary tract infections, eye infections resulting in the loss of sight and secondary infections of wounds especially burned tissue.

•Systemic infections including pneumonia have resulted from infection by P. aeruginosa following organ transplant.

•P.aeruginosa is resistant to many antibacterial agents and is biochemically very versatile, being able to use many disinfectants as food sources.

 2.1.2: Pseudomonas fluorescens: is a well-known producer of a yellow-green pigment. 

2.2: Enterobacteria:

Characteristics: They are facultatively anaerobic, Gram-negative rods that inhabit the human intestines and animals; hence the name Enterobacteria.

• All enterobacteria have peritrichous flagella.

•Over 25 genera of enterobacteria are recognized, many with pathogenic importance.

•Members of the enterobacteria group are members of the family Enterobacteriacae.

•Many selective and diagnostic media and differential biochemical reactions are available to isolate and distinguish members of this group, as they are of great significance in public health.

2.2: Enterobacteria  members cont’d

2.2.1: Escherichia coli: is a cause of enteritis in young infants and the young of farm animals, where it can cause diarrhoea and fatal dehydration.

•It is a common infectant of the urinary tract and bladder in man, and is a cause of pyelitis, pyelonephritis and cystitis. 

2.2.2:  Salmonella: species that cause intestinal disease known as salmonellosis: Among the species are:-

2.2.2.1: Salmonella typhi: is the causal organism of typhoid fever.

2.2.2.2:  Salmonella paratyphi: causes partyphoid fever.

2.2.2.3: Salm.typhimuriium and Salm. Enteritidis and other closely related organisms are a cause of bacterial food poisoning.

2.2: Enterobacteria members cont’d

2.2.3: Shigella:

Characteristics: facultatively anaerobic, gram-negative rods.

These species cause intestinal disease known as shigellosis.

Members: Among the species are:-

2.2.3.1: Shigella shiga

2.2.3.2: Sh. flexineri

2.2.3.4: Sh.sonnel

2.2.3.5: Sh. boydii

•All species are the causes of bacillary dysentery

2.2: Enterobacteria members cont’d

2.2.4: Klebsiella:

Characteristics: facultatively anaerobic, gram-negative rods.

Members:

2.2.4.1: Klebsiella pneumoniae sub-species aerogenes is found in the gut and respiratory tract of man and animals, and in soil and water.

•It can give rise to acute bronchopneumonia in man but is not a common pathogen.

•Other Klebsiella species can cause intestinal disease and other infections.

2.2.5: Proteus:

Members:

2.2.5.1: Proteus vulgaris and Pr. Morganii can infect the urinary tract of man.

•They are avid decomposers of urea, producing ammonia and carbon dioxide.

•These organisms occasionally cause wound infection. 

2.2.6: :Serratia

2.2.6.1: Serratia marcescens:

•This very small organism, 0.5-1.0µm long, is used to test bacterial filters.

•It is not to be regarded as non-pathogenic, although infections arising from it are rare.

2.3: Vibrio:

Characteristics: Vibrios are curved, Gram-negative, facultatively anaerobic rods. They belong to the family Vibrionaceae.

Members:

2.3.1: Vibrio cholerae, is the cause of cholera in humans.

2.3.2: V. parahaemolyticus occurs in sea water and has been implicated in food poisoning following consumption of raw fish.

2.4. Pasteurellas:

Characteristics: are facultatively anaerobic, gram-negative-rods.

•They belong to the family Pasteurellaceae.

•They are distinguished from vibrios and enterobacteria by their small size and inability to move.

•The genera Pasteurella, Haemophilus, and Actinobacillus are among the important members of the group.

•Members:

2.4.1: Haemophilus influenzae: is a cause of meningitis in children and conjunctivitis and is one of the most important causes of chronic bronchitis.

2.4.2: Pasteurella multocida causes cholera in fowl.

2.5. Brucella:

Characteristics: gram-negative, aerobic rods found in many domesticated animals and in some wild animals as parasites, causing condition known as brucellosis. Also found in soil and water.

Members:

2.5.1: Brucella abortus: is a cause of spontaneous abortion in cattle. In man it causes undulant fever, i.e. a fever in which temperature undulates with time.

2.5.2: B. melitensis: infects goats; it causes an undulant fever called Malta fever, common in mediterranean countries where large flocks of goats are kept.

2.5.3: B. suis: is found in pigs; manifests itself in man as undulant fever & occurs frequently in North America.

2.6. Bordetella:

Characteristics: gram-negative, aerobic rods.

Members:

2.6.1: Bordetella pertusis: is the cause of whooping-cough, a disease which is eradicated by immunisation.

2.7: Yersinia:

Characteristics: are facultatively anaerobic, gram negative rods.

Members:

2.7.1: Yersinia pseudotuberculosis sp. Pestis (formerly Pasteurella pestis) is the causal organism of plague or the black death.

•It infects the lymphatic system to give bubonic plague, or the respiratory system, giving the rapidly fatal pneumonic plague.

2.8. Bacteroides:

Characteristics: anaerobic bacteria having unique motility and flagellation patterns.

•They occur in the alimentary tract of man & animals.

•  Several species digest cellulose in the rumen of the cow and thereby break down plants.

•Human feces contains large numbers of bacteria belonging to the genus Bacteroides, which may be helpful in digestive processes.

Members:

2.8.1: Bacteroides fragilis: is a possible cause of human blood & wound infections especially after surgery.

2.9. Camplobacter:

Characteristics: Camplobacters are thin, gram-negative, rods but often appear in culture with one or more spirals or as S and W (gull-winged) shaped cells.

•They are anaerobic or microaerophilic (i.e. grows best in environment with less oxygen than is found in air).

•They are unable to grow below 30°C and move by a single polar flagellum.

•Members:

2.9.1: Camplobacter jejenum: is a cause of enteritis in man and is mainly transmitted by contaminated food; i.e. is a food-poisoning microorganism.

2.10: Chlamydia:

Characteristics: are extremely tiny bacteria, below the resolving power of the light microscope, (submicroscopic).

•They are rod-shaped and coccoid organisms.

•Although the Gram stain is not used for identification, the bacteria have cell walls containing muramic acid resembling those in Gram-negative bacteria. 

•Chlamydiae display a growth cycle that takes place within host cells.

•The bacteria invade the cells and differentiate into dense bodies called reticulate bodies.

The reticulate bodies reproduce and eventually form new chlamydiae in the host cell called elementary bodies.

2.10: Chlamydia cont’d

Members:

2.10.1: Chlamydia trachomatis:  cause several diseases in humans, such as trachoma, a disease of the eye, conjunctivitis and non-gonococcal urethritis, an infection of the reproductive tract. It is sensitive to the rifampicin, the tetracyclines and erythromycin.

2.10.2: Chlamydia psittaci: causes psittacosis, a disease of the lung tissues and occurs mainly in the parrot family (hence psittacosis) and other avian species, (hence ornithosis).

•The disease is often found in persons who work in pet shops selling parrots and can be fatal.

2.11. Rickettsiae.

Characteristics: are rod-shaped and coccoid bacteria with the characteristics of Gram-negative cell walls. 

•They share with chlamydias the property of growing only in living tissue.

•They are obligate intracellular parasites that infect humans as well as arthropods such as ticks, mites, and lice.

•These bacteria cannot be seen with the light microscope,  i.e. submicroscopic.

•Rickettsiae divides by binary fission and may be cultivated in the blood of laboratory animals or in the yolk sac of the embryo of the domestic fowl, and it is by this method that the organism is grown to produce vaccines. 

2.11. Rickettsiae cont’d

•Rickettsiae are very important as human pathogens since it gives rise to a variety of typhus infections in man, the intermediate carriers being lice, fleas, ticks or mites and can occur without harm to these arthropod hosts.

Members:

Among  the various species that cause  diseases are:

2.11.1: Rickettsiae prowazekii : causes epidemic typhus.

2.11.2: Rickettsiae quintana: causes trench fever or rocky Mountain spotted fever.

2.11.3: R. typhi: causes murine typhus , endemic typhus &  scrub typhus

2.11.4: Coxiella burnetii: causes Q-fever.

2.12. Legionella:

Characteristics: gram-negative, aerobic rod-shaped.

• It grows in water supplies especially water maintained in storage tanks.

•Members:

2.12.1: Legionella pneumophila: is a causal organism of Legionnaire’s disease or legionellosis.

•It causes an influenza-like fever which is accompanied by pneumonia.

3.1. Staphylococci:

Characteristics: are Gram-positive, spherical-shaped bacteria that divide in planes to produce clusters (like bunch of grapes) or packets.

•The organisms are non-motile and non-sporing.

•They grow aerobically or anaerobically.

Members:

3.1.1: Staphylococus aureus: produces a golden yellow pigment and is a cause of skin lesions such as boils, abscesses, impetigo and carbuncles, especially if they produce the enzyme coagulase, which causes blood clotting. 

3.1. Staphylococci cont’d

Members:

3.1.1: Staphylococus aureus:

•Also is associated with mucous membranes lesions, pneumonia, conjunctivitis, staphylococcal meningitis and can affect bone tissue leading to staphylococcal osteomyelitis.

•Staphylococcus aureus is involved in cases of food poisoning and toxic shock syndrome, i.e. it produces a toxin which, if ingested with food in which the organism has been growing, can give rise to food poisoning.

•A common manifestation of its infection is the production of pus, i.e. the organism is pyogenic.

3.2. Streptococcus:

Characteristics: are gram-positive, non-sporing, spherical organisms which grow in chains like a string of beads, and can grow aerobically or anaerobically. On blood agar, certain species partly destroy the red blood cells and are said to be alpha-hemolytic. Other species completely destroy the blood cells and are beta-hemolytic. Those streptococci producing no blood cell destruction are gamma-hemolytic.

Members:

3.2.1: Streptococcus pyogenes: is extremely dangerous pathogen; it produces a series of toxins, including erythrogenic toxin which induces a characteristic red rash, and a family of toxins which destroy the formed elements of blood.

3.2. Streptococcus members cont’d:

•Typical diseases caused by Str.pyogenes are scarlet fever and acute tonsillitis (sore throat), and the organism is a dangerous infective agent in wounds and in blood poisoning after childbirth (puerperal sepsis).

•Rheumatic fever and acute inflammation of the kidney are serious sequelae of streptococcal infection.

•Other species are associated with dental caries.

3.2.2: Streptococcus pneumoniae:  These are diplococcus organisms; i.e. they grow in pairs.

•is the causal agent of acute lobar pneumonia and also of meningitis, peritonitis and conjunctivitis.

•Harmless strains of streptococci are used in the production of yogurt, buttermilk, and cheese.

4.Gram‐Negative Cocci Bacteria (Spherical-shaped)

4.1. Neisseria:

Characteristics: gram-negative pathogenic cocci, slightly curved rather than true spheres and have been likened to a kidney bean in shape.

•They occur in pairs and embedded in pus cells.

Members:

4.1.1:Neisseria gonorrhoeae: is the causal organism of the venereal disease gonorrhoea. The organism can also affect the eyes, causing a purulent ophthalmia.

4.1.2: Neisseria meningitidis:  is a cause of cerebro-spinal fever or meningococcal meningitis.

4.2. Branhamella.

•Branhamella catarrhalis (formerly N.catarrhalis) is a harmless member of the genus and is often isolated from sputum.