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Gram Positive and Gram Negative Bacilli of Medical Importance

General Introduction 

Bacteria (singular: bacterium) constitute a large domain of prokaryotic microorganisms. 
Found in water, soil, sewage, intestines of humans e.t.c.
0.1-10um long. 
Simple cell structure
Multiply by binary fission. 
Classified By: Morphology, Staining reactions, Culture characteristics, Biochemistry, Antigenic structure & genetic Composition.

General Morphology of Bacteria

 Cocci (spherical) 
 Bacilli (Rod-shaped)
 Vibrio, (slightly curved rods or comma-shaped).
 Others: spiral-shaped, called spirilla or tightly coiled, called spirochaetes.

Many bacteria can exists as:
     single cells, others associate in characteristic patterns:
     diploids (pairs) e.g. Neisseria 
     chains, e.g. Streptococcus
     Staphylococcus group together in “bunch of grapes”.
     Bacteria can also be elongated to form filaments e.g. the  Actinobacteria.

Gram-Negative Bacteria                                              Gram-Positive Bacteria
Lipopolysaccharide exterior cell wall 
which is thin.                                                             A thick peptidoglycan layer present.
Do not retain the crystal violet dye
and react only with a counter stain.            
These generally stain pink.                          Gram-positive bacteria retain the crystal violet dye and                                                                  change into purple during staining identification
No teichoic acids present.                          Teichoic acids are present.

                                   Representation of Gram Positive Cocci Bacteria

Gram Positive Bacilli of Medical Importance






Gram positive straight or curved rods.

Found in soil, plant and animals.
Species include: C. diphtheriae, C. ulcerans, C. haemolyticum, C. pygenes, C. ovis. 

C. diphtheriae is a Gram positive long, thin and curved rods that appear in clusters. 
non-motile, non-sporing, catalase and nitrate positive, oxidase negative, and ferments glucose and maltose.

Four (4) biotypes are known:

C. diphtheriae mitis, 
C. diphtheriae intermedius, 
C. diphtheriae gravis, and
C. diphtheriae belfanti.

Pathogenicity and Treatment of C. diphtheriae

Infection is by inhaling respiratory droplets or by contacts with contaminated objects causing the following:

Cutaneous diphtheria
Nasal, nasopharyngeal and tonsilar diptheriae especially in young children.
Acute circulatory failure, pneumonia and otitis media. 
Treatment involves the use of antitoxin to neutralize C. diptheriae toxin and antibiotics such as penicillin, erythromycin, vancomycin can be administered.


  A genus of Gram-positive bacteria that are rod shape, obligate  anaerobes capable of producing endospores. 
 Catalase negative,
 Oxidase negative.

  Four main species responsible for disease in humans:
 C. botulinum
 C. difficile, 
 C. tetani
 C. perfringens

C. botulinum is found in soil, water and sewage causing a food borne disease called botulism, neurotoxin is ingested in contaminated food.  The toxin causes paralysis 12-36hrs after ingestion. 

C. difficile  can flourish when other bacteria in the gut are killed during antibiotic therapy, leading to pseudomembranous colitis (a cause of antibiotic-associated diarrhea).

C. tetani causes tetanus, a serious fatal disease caused by the neurotoxin –tetanospasmin, produced by C. tetani. it causes muscular rigidity and spasms, lock jaw & arching of the back due to contractions of back muscles. A toxoid vaccine is available to prevent the disease.

C. perfringens bacteria are the third most common cause of foodborne illness, with poorly prepared meat and poultry, or food properly prepared but left to stand too long. 

 Five (5) types of C. perfringens (A-E) are known. Human disease is caused by types A and C. 

C. perfringens type A1: Causes Gas gangrene (myonecrosis), puerperal infection, septicaemia.

C. perfringens type A2: Causes food-poisoning 8-12 hrs of ingesting contaminated meat. Symptoms are similar to V.cholerae. Most infections are self-limiting.

C. perfringens type C: Causes a severe form of jejunities (necrotizing enterocolitis), known as ‘pigbel’ from insufficiently cooked pig meat. 

Diagnosis of C. perfringens.

C. perfringens can be diagnosed by Nagler's Reaction where the suspect organism is cultured on an egg yolk media plate. 
    One side of the plate contains anti-alpha-toxin, while the other side does not. 

A streak of suspect organism is placed through both sides. An area of turbidity will form around the side that does not have the anti-alpha-toxin, indicating uninhibited lecithinase activity.


 Large aerobic spore-bearing rods. 
Most species are motile, some non-motile.

The important species that may cause infections are B. anthracis, B. cereus and B. subtilis.
The main pathogen of the group is B. anthracis (anthrax bacillus).
B. anthracis causes anthrax- a disease of sheep, cattle, goats and other herbivores. 
Humans becoming infected after contacts with infected animals or their skins. 

Pathogenicity of B. Anthracis

Cutaneous anthrax, the most common form (95%), causes a localized, inflammatory, black, necrotic lesion (eschar) when the bacilli enters a damaged skin.

Pulmonary anthrax caused by inhaling large numbers of B. anthracis spores. It is characterized by sudden, massive chest edema followed by cardiovascular shock.

Enteric anthrax: Due to ingestion of infected meat. Gastroenteritis with fever, abdominal pain, and bloody diarrhoea occurs. Septicaemia often develops.

Prevention and treatment of B. Athracis

Workers at risk of infection should be vaccinated. 

B. anthracis can be treated with β-lactam antibiotics such as penicillin, and others which are active against Gram-positive bacteria. 


Mycobacteria are slender, straight, or slightly curved, non-motile rods, aerobic, non-capsulated and non-sporing.

Mycobacteria of Medical Importance include:
M. tuberculosis
M. leprae
M. ulcerans
M. bovis
M. africanum 

Mycobacterium Tuberculosis

M. tuberculosis is a non-sporing, non-capsulated straight or slightly curved slender rod, measuring 1-4um x 0.2-0.6um. Although it does not Gram stain well due to its waxy surface. 
Major cause of tuberculosis in man. 
It is found in infected humans and is transmitted to primates, dogs, and other domestic animals.
Best demonstrated using the Ziehl-Neelsen staining technique or a fluorescence technique.

Pathogenicity of M. Tuberculosis

Highly contagious and kills more people than any other infectious disease.

The main route of entry is by inhalation of droplets or dust particles containing the bacilli. 

     It can occur in different forms:

   Pulmonary tuberculosis: Occurs when the primary infection does not heal completely and there is continued multiplication or there is reactivation of organisms in the lung several months or years later due to poor health, malnutrition or defective immune responses. 

Symptoms are loss of weight, fever, night sweats, fatigue, chest pain, and anaemia. Chronic cough with production of mucopurulent sputum which may contain blood (haemoptysis). 
Miliary tuberculosis: Infection occurs if a site of primary infection ruptures through a blood vessel and bacilli are disseminated throughout the body. Small granulomata are formed which on a chest X-ray, look like millet seeds (hence name military tuberculosis). Hepatomegally, splenomegly and enlargement of the lymph glands.

Renal and urogenital tuberculosis: Tubercle bacilli reach the kidneys and genital tract through blood circulation, usually some years following primary tuberculosis. Tuberculosis of the genital tract (epididymitis in males, endometrial tuberculosis in females) can cause infertility and PID.

  Bone and Joint Tuberculosis: A commonly infected site is the spine which may lead to the collapse of vertebrae and the formation of a ‘cold’ abscess in the groin. This form of the disease is rare.

Gram Negative Bacilli of Medical Importance

 There are many Gram-negative bacilli of medical significance.

The most important of these are members of the family Enterobacteriaceae. Other genera of medical importance include Vibrio, Campylobacter and Pseudomonas.

The family Enterobacteriaceae is the largest and most heterogeneous collection of medically important gram-negative bacilli and are commonly isolated from clinical specimens.

    Over fourteen genera have been described to cause human infection but by far the most important single species is Escherichia coli.

Other important genera include Salmonella, Shigella, Proteus, Klebsiella, Enterobacter, Citrobacter, Serratia and Yersinia.


Enterobacteriaceae are found in soil, water, and vegetation and are part of the normal enteric flora of all animals including humans.

Some members of the family e.g. Shigella, Salmonella, Yersinia pestis are almost always associated with disease when isolated from humans i.e.,

They are never “normal” flora, whereas others e.g. Escherichia coli, Klebsiella pneumoniae, and Proteus mirabilis are members of the normal commensal flora and usually cause opportunistic infections.

 More than 5% of hospitalized patients develop hospital acquired (nosocomial) infections, with members of the Enterobacteriaceae responsible for a large proportion of these infections.

Laboratory Features

Members of this family are aerobic and facultative anaerobic non-sporing Gram-negative bacilli, and are either motile or are non-motile.

Some have the ability to ferment glucose, oxidase negative and reduce nitrates to nitrites.

They are generally not difficult to cultivate and have simple nutritional requirements.

Escherichia coli

  Escherichia coli are Gram negative straight rods measuring 1-3x0.4-0.7um and arranged singly or in pairs.

They are motile, nitrite positive, ferment lactose, indole positive.

They form part of the normal bacterial flora of the gastrointestinal tract of man and animals.
 They may be found in water, soil and vegetables.

Pathogenicity of E.coli

Urinary tract infections: Commonest pathogen isolated from patients with cystitis, pyelitis and pyelonephritis.

Causes wound infection, peritonitis and infection of the gall bladder.

Diarrhoea in children under three years and very elderly adults. It also causes traveller’s diarrhoea.

   The diarrhoea causing strains of E.coli fall into 5 groups:
 Enteropathogenic E.coli (EPEC): Associated with outbreaks of infantile diarrhoea in bottle fed babies.

 Enterotoxin producing E. coli (ETEC) are the chief cause of acute watery diarrhoea in the tropics. They also cause traveller’s diarrhoea.

   Enteroinvasive (EIEC): Cause a dysentery-like diarrhoea with blood and mucus in stool like shigellae. They penetrate and multiply within the cells of the intestinal epithelium while destroying them and consequently cause ulceration.

  Enterohaemorrahgic E. coli strains cause hemolytic ureamic syndrome (HUS) and heamorrhagic colitis associated with bloody diarrhoea.

  Enteroaggregative E. coli: Causes chronic watery diarrhoea and vomiting, mainly in children. The bacteria adhere to tissue cells in stacks.
     Antimicrobials used in the treatment of E-coli urinary and other infections include sulphonamides, trimethoprim, nitrofurantoin, cephalosporins e.t.c. In the treatment of E-coli diarrhoea, the use of antibiotics is of minor importance compared to the rehydration of patient.

Genus Salmonella

 Gram negative rods, motile and facultative anaerobes.
 Oxidase negative, catalase positive, Hydrogen Sulphide positive.
 Non-lactose fermenters with pale coloured colonies on DCA.

Pathogenically, they can be found in faeces, blood, and urine of patients.

Cause gastrointestinal diseases.

They are grouped together in the species S. enterica and are distinguished from each other serologically into approximately 2200 different serotypes.
Most common ones are S.typhi, S. paratyphi A, B, C.

S. typhi is water-born while the paratyphis are food-born.

 Pathogenicity of Salmonella

Enteric fever: Infection is by ingesting the organisms in contaminated food or water or contaminated hands. Persistent high fever, severe headache, splenomegally, nausea, mental confusion are the symptoms of infection caused by S.typhi (most serious form) and S.paratyphi A, B, C.

Enterocolitis: Sources of infections are poultry, meat, and meat products, eggs e.t.c. Symptoms include diarrhoea, vomiting, fever, abdominal pain which occurs 12-36hrs after eating infected food.

Antimicrobials with activity against S.typhi are co-trimoxazole and ampicillin.

Genus Shigella

Gram negative, non-sporing, and non capsulate rods.
Shigella are non-motile unlike salmonellae.
Hydrogen sulphide negative, lactose negative, urease negative, citrate negative.

These are divided into four (4) serogroups:
    Group A: Shigella dysenteriae
    Group B: Shigella flexneri
    Group C: Shigella boydii

    Group D: Shigella sonnei

Pathogenicity of Shigella

Shigella species cause shigellosis (bacillary dysentery). About 60% of infections are due to S. flexneri, 15% to S. sonnei, 6% to S. boydii, and 6% to S. dysenteriae.

 Transmission is mainly by faecal-oral route with poor sanitation, unhygienic condition and overcrowding.

Houseflies are also vectors of shigellae.
Symptoms are dehygdration, protein loss, ulceration of the intestinal tract with severe dysentery, rectal pain, abdominal cramps, high fever.

Death can occur from circulatory collapse or kidney failure.

Proteus Mirabilis and Enterobacter species

Proteus Mirabilis: Are motile, non-capsulate, gram negative pleomorphic rods. It is the main Proteus species of medical importance.
It causes urinary infections, abdominal and wound infections. Non lactose fermenters, Beta-galactosidase negative.

Enterobacter species: found in the intestinal tract of humans and animals, in soil, sewage and water. Opportunistic pathogens associated with urinary infections, wound infections especially in persons already in poor health.

Genus Vibrio 

Many species are motile, aerobic Gram-negative rods that are found in aquatic environments. 

Three species are commonly pathogenic for humans- (V.parahemolyticus, V. vulnificus, V. Cholerae)  the most important being V. cholerae – the etiologic agent of epidemic cholera. 
Vibrio parahemolyticus is a marine vibrio that causes gastroenteritis sssociated with the consumption of raw seafood.

Vibrio vulnificus is a rare, but often fatal cause of septicemic wound infections associated with sea water. 

 Vibrio Cholera is an acute intestinal infection causing profuse watery diarrhea, vomiting, circulatory collapse and shock. 
    If left untreated, 25-50% of typical cholera cases are fatal and death can result following dehydration within hours.

                         Who Gets Cholera?
A person can get cholera by drinking water or eating food contaminated with the cholera bacterium. Brackish and marine waters are a natural environment for the etiologic agents of cholera, Vibrio cholerae O1 or O139. 

Epidemics are often related to fecal contamination of water supplies or street vended foods. Disease is occasionally transmitted through eating raw or undercooked shellfish that are naturally contaminated.

Genus Pseudomonas

A group of strictly aerobic, non-sporing, Gram negative bacilli and  motile organisms. 
They are found in water, soil and sewage.
The species include Ps. aeruginosa, Ps. mallei, Ps. Putida, Ps. Fluorescens.

                      Pseudomonas aeruginosa

P. aeruginosa is an obligate aerobe. 
Found in the intestinal tract, water, soil and sewage. Oxidase positive. Produces a blue-green pigment and a yellow-green pigment. 

Cultures have distinct smell due to the production of 2-aminoacetophenone.

Frequently found in moist environments in hospitals, (sinks, cleaning buckets, drains, e.t.c) and can grow in saline and other aqueous solutions. 

 Therefore many infections with P. aeruginosa are hospital-acquired, affecting those already in poor health and immunosuppressed. 

P. aeruginosa typically infects the pulmonary tract in patients with cystic fibrosis, urinary tract following catheterization, burns, wounds, and also causes other blood infections.

Treatment of P. aeruginosa

P. aeruginosa is naturally resistant to a large range of antibiotics and may demonstrate additional resistance after unsuccessful treatment.

Antibiotics that have activity against P. aeruginosa may include:
    aminoglycosides quinolones and cephalosporins.

 Adequate health education, and sensitization measures should be adopted for the citizenry in order to curb the menace of outbreaks of some of these organisms.


  Personal hygiene and maintaining a clean aerated environment is of utmost importance in ensuring a sick-free lifestyle for the home, community and the nation at large.


Collier,L, Balows, A (2013) Topley and Wilson’s Microbiology and Microbial infection, pp.334-445. Monarth press, florida.
J Ochie, A. Kolhatkar (2008).Medical Laboratory Science Theory and Practice. Tata McGraw-Hill, New Delhi.
Monica Cheesbrough, (2004) District Laboratory Practice in Tropical Countries, Part2. Sheck Wah Tong Press, Hong Kong. 

Thank you for reading.


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