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288 Quick Review Series: BDS 2nd Year
Part II
Systemic Bacteriology
Topic 16
Staphylococcus, Streptococcus, Pneumococcus
and Neisseria
LONG ESSAYS
Q. 1. Describe the morphology, staining characters and 5. MacConkey’s medium: Due to fermentation of lactose
pathogenicity of Staphylococcus. Add a note on labora- the colonies on the MacConkey’s medium are pink in
tory diagnosis of staphylococcal infections. colour and are very small about pinhead size.
6. Liquid medium: In liquid medium uniform turbidity is
Ans.
produced.
MORPHOLOGY OF STAPHYLOCOCCUS 7. Selective media: Several selective media are used for
isolating S. aureus from specimens such as faeces. These
AUREUS include following media:
1. They are spherical cocci, approximately 1 mm in diam- a. Salt milk agar or salt broth (media containing NaCl)
eter, arranged characteristically in grape-like clusters b. Ludlam’s medium (media containing lithium chlo-
due to cell division occurring in three planes, with ride and tellurite)
daughter cells tending to remain in close proximity. c. Media containing polymyxin
2. They may be found singly, in pairs and in short chains
of three or four cells. Long chains never occur.
PATHOGENICITY
3. They are Gram-positive, nonsporing, nonmotile, non-
capsulated, aerobic and normally facultative anaerobic. 1. Staphylococcus produces two types of diseases as follows:
a. Infections
b. Intoxications
CULTURAL CHARACTERISTICS 2. In the infections cocci gain access to damaged skin,
1. They grow readily on ordinary media within a tempera- mucosal or tissue sites colonized by adhering to cells or
ture range of 10–42°C, the optimum temperature being extracellular matrix, evade host defence mechanisms
37°C and pH 7.4–7.6. multiply and cause tissue damage.
2. On nutrient agar after incubation for 24 h, S. aureus pro- 3. In intoxications the disease is caused by the bacterial toxins
duces large around 2–4 mm diameter, circular, convex, produced either in the infected host or preformed in vitro.
smooth, raised, shiny, opaque and easily emulsifiable
colonies.
STAPHYLOCOCCAL DISEASES
a. Though some of them may be white, orange or yel-
low, most strains produce golden yellow pigment. S. aureus is an important pyogenic organism and its lesions
b. The pigment production occurs optimally at 22°C are characteristically localized in nature.
and only in aerobic cultures and it can be enhanced Common staphylococcal infections are as follows:
by incorporation of 1% glycerol monoacetate or
milk in the medium.
A. Skin and Soft Tissue Infections
3. Nutrient agar slope: The growth on nutrient agar slope is
confluent presenting a characteristic oil paint appearance. 1. These may be of varying severity and are important
4. Blood agar: Colonies of S. aureus on blood agar are cause of hospital acquired infection.
similar to those on the nutrient agar. Most strains are 2. They include folliculitis, furuncles (boil), styes, boils, ab-
haemolytic especially when incubated under 20–25% scesses, carbuncles, impetigo and pemphigus neonatorum.
CO2 and they produce marked haemolysis on rabbit or 3. Sepsis in wounds and burns is also usually caused by
sheep blood and weak on horse blood agar. the S. aureus.
,Click here to Visit - www.thedentalhub.org.in
Section | III Microbiology 289
B. Deep Infections 2. The underlying pathogenic mechanism is the stripping
of the superficial layers of the skin from the underlying
i. Musculoskeletal infections
tissues by the exfoliative or epidermolytic toxin.
a. Acute osteomyelitis is the commonest deep infection,
3. The severe form of SSSS is known as Ritter’s disease in the
the bacteria being implicated as the cause in 90% cases.
newborn and toxic epidermal necrolysis in older patients.
b. Tropical pyomyositis is formation of multiple ab-
4. Milder forms are bullous impetigo and pemphigus neo-
scesses in the voluntary muscles observed usually in
natorum.
young adults.
c. Arthritis and bursitis
ii. Respiratory infections: In the respiratory tract, it causes E. Toxic Shock Syndrome
tonsillitis, pharyngitis, sinusitis, otitis, bronchopneumo- Toxic shock syndrome is a potentially fatal multisystem
nia, lung abscess, empyema and rarely pneumonia. disease produced by S. aureus strains usually presenting
iii. Central nervous system: Abscess, meningitis, intracra- with fever, hypotension, myalgia, vomiting, diarrhoea, mu-
nial thrombophlebitis. cosal hyperaemia and an erythematous rash which desqua-
iv. Endovascular infections: Bacteraemia, septicaemia, py- mates subsequently. This is associated with infection of
aemia, and endocarditis. Staphylococcal septicaemia is mucosal or sequestrated sites by the toxic shock syndrome
rare but a serious condition. toxin (TSST) producing S. aureus.
v. Urinary infections: Staphylococci are uncommon in 1. Staphylococcal enterotoxins and toxic shock syndrome
routine urinary tract infections, though they do cause toxin-1 (TSST-1) are superantigens and potent activa-
infections in association with local instrumentation, tors of T lymphocytes.
implants or diabetes. Renal abscess may be produced 2. They stimulate very large number of T cells without rela-
due to haematogenous spread. tion to their epitope specificity. This leads to an excessive
and dysregulated immune response with release of cyto-
C. Staphylococcal Food Poisoning kines IL-1 and IL-2, tumour necrosis factor, and interferon-
gamma. This is responsible for multisystem involvement.
1. Staphylococcal food poisoning results on consumption 3. It was first observed in 1978 in children and adolescents
of food contaminated with enterotoxin producing staph- but showed wide outbreaks in 1980 in menstruating
ylococci. women using highly absorbent vaginal tampons.
2. The bacteria produce this toxin when grown in carbohy- 4. Convalescents show TSST–1 antibody is protective and
drate and protein foods. Common food items usually absence of it is a factor in the pathogenesis of the condition.
involved are meat, fish, milk and milk products.
3. Sufficient time should elapse between the contamina-
LABORATORY DIAGNOSIS
tion of the food and ingestion of the contaminated food
so that the organism can produce enough enterotoxin. Specimen to be collected depends on type of lesion as
The diarrhoea and vomiting sets in within 6 h of ingest- follows:
ing the contaminated food. 1. Pus from suppurative lesions
a. Enterotoxin is a heat-stable toxin and is also resistant 2. Sputum for respiratory infections
to action of GIT enzymes. 3. CSF for meningitis
b. Eight antigenic types of enterotoxins are currently 4. Blood for septicaemia
known, named A, B, C1–3, D, E and H of which type A 5. Suspected food, vomit or feces for food poisoning
is responsible for most of the cases.
Diagnosis is made by the following:
c. The toxin is believed to act directly on autonomic
nervous system to cause the illness, rather than on GI
a. Direct microscopy
mucosa.
d. Ingestion of enterotoxin as little as microgram amounts 1. Examination of a Gram-stained smear of pus or other
can cause illness like vomiting and diarrhoea in humans. specimen may show Gram-positive cocci in clusters
The emetic effect is brought about by the stimulation of with some single and paired cocci.
the CNS through the neural receptors in the GIT. 2. This is useful for pus and not for other specimen such
e. The toxin also exhibits pyrogenic, mitogenic, hypo- as sputum or feces where mixed bacterial flora is nor-
tensive, thrombocytopenic, and cytotoxic effects. mally present.
b. Culture
D. Exfoliative Diseases
Diagnosis is readily made by culture.
1. Exfoliative toxin is also known as ET or exfoliatin and
is responsible for the staphylococcal scalded skin syn- 1. The specimen is plated on the nutrient agar or blood
drome (SSSS). agar and incubated overnight. S. aureus colonies are
,Click here to Visit - www.thedentalhub.org.in
290 Quick Review Series: BDS 2nd Year
identified as large, circular, convex, smooth, raised, 2. It is exacting in nutritive requirements and growth oc-
shiny and opaque colonies on both the media. curs only in media enriched with blood or serum or
2. Specimens such as feces for food poisoning, swabs media containing fermentable carbohydrates.
from carriers where the bacteria are scanty, are inocu- 3. On blood agar, after incubation the colonies are small,
lated on selective media like Ludlam’s or salt-milk agar circular, semitransparent, convex discs with beta hae-
or Robertson’s cooked meat medium containing 10% molysis.
NaCl. 4. Growth and haemolysis are promoted by 10% CO2.
3. The smears from the culture media are examined under Virulent strains on fresh isolation from lesions produce
the microscope and biochemical tests done to confirm a matt or finely granular colonies while avirulent strains
the S. aureus. form glossy colonies. Strains with well-marked cap-
sules form mucoid colonies.
c. Coagulase test
PATHOGENESIS
1. Coagulase test is done by two methods as follows:
a. Tube coagulase test 1. Streptococcus pyogenes adheres to the pharyngeal epi-
b. Slide coagulase test thelium by means of lipoteichoic acid covering the sur-
2. Tube coagulase test detects free coagulase. The slide face epithelial cells.
coagulase test detects bound coagulase and usually 2. The infection may spread to the surrounding tissues
gives results parallel with tube test. leading to suppurative complications like otitis media,
3. When there is a divergence, the tube test will be decid- mastoiditis, quinsy, Ludwig’s angina and suppurative
ing factor. adenitis. Meningitis may occur rarely.
3. Scarlet fever occurs when the infecting strain of S. pyo-
genes produces erythrogenic toxin in a patient usually a
d. Antibiotic sensitivity tests child with no antitoxic immunity leading to a combina-
1. Should be done simultaneously as a guide to treatment. tion of sore throat and a generalized erythematous rash.
This is important as staphylococci readily develop resis-
tance to drugs.
LABORATORY DIAGNOSIS
Specimens collected are pus, throat swab, sputum, genital
e. Serological tests swabs, blood and CSF. Specimens can be transported in
1. These may sometimes help to diagnose hidden deep Pike’s medium.
infections. 1. Microscopy: The presence of Gram-positive cocci in chains
2. ASO titre more than 2 units/mL with rising titre is of under Gram’s staining is indicative of Streptococcus
value in diagnosing deep seated infections such as bone infections. But smears from throat and genitalia are of no
abscess. value, where Streptococcus forms part of normal flora.
2. Culture: The specimen is plated on blood agar and in-
Q. 2. Write in detail about the morphology, pathogene- cubated at 37°C anaerobically or under 5–10% CO2.
sis, laboratory diagnosis and the treatment of Strepto- The growth is identified by beta haemolysis, Gram’s
coccus pyogenes. staining and antibiotic sensitivity. S. pyogenes is more
Ans. sensitive to bacitracin, hence, a filter paper disc dipped
in a solution of bacitracin (1 unit/mL) is applied on the
MORPHOLOGY OF STREPTOCOCCUS surface of an inoculated blood agar. After incubation,
1. They are Gram-positive cocci arranged in chains or wide zone of inhibition is seen with S. pyogenes, but not
pairs. Individual cocci are spherical or oval in shape, with other streptococci.
measuring 0.5–1.0 mm in diameter. 3. Serology
2. Chain formation is due to cocci dividing in one plane a. Haemolytic streptococci are grouped by Lancefield’s
only and daughter cells failing to separate completely. method. The fluorescent antibody technique has
3. They are nonmotile and nonsporing. Some strains of S. pyo- been employed for rapid identification of group A
genes and some group C strains possess capsules made of streptococci.
hyaluronic acid while polysaccharide capsules are found in b. In poststreptococcal lesions ASO test is used. ASO
groups B and D. Capsules are best seen in young cultures. test is used for retrospective diagnosis of rheumatic
fever and glomerulonephritis.
c. Streptolysin O is antigenic and antistreptolysin ap-
Cultural Characteristics pears in sera following streptococcal infections. Es-
1. It is an aerobe and facultative anaerobe growing best at timation of this antibody known as antistreptolysin,
the temperature of 37°C and an optimum pH of 7.2–7.4. is used as a standard serological procedure for
, Click here to Visit - www.thedentalhub.org.in
Section | III Microbiology 291
retrospective diagnosis of poststreptococcal compli- a convenient, specific, sensitive screening test. It becomes
cations. positive after all types of streptococcal infections.
d. ASO titres higher than 200 are indicative of poststrep-
tococcal infection. High levels are found in acute
TREATMENT
rheumatic fever, but in glomerulonephritis, titres are
low. 1. All beta haemolytic group A streptococci are sensitive
e. DNAase B test is also used. Titres of higher than 300 to penicillin G and most are sensitive to erythromycin.
are taken as significant. 2. In patients allergic to pencillin, erythromycin or cepha-
4. Streptozyme test lexin may be used.
A passive slide haemagglutination test using RBC sen- 3. Antimicrobial drugs have no effect on established glo-
sitized with crude extracellular antigens of Streptococcus is merulonephritis and rheumatic fever.
SHORT ESSAYS
Q. 1. Lesions caused by Staphylococcus aureus Exfoliative Diseases
Ans. Common staphylococcal infections are as follows: l Exfoliative toxin is also known as ET or exfoliatin and
is responsible for the staphylococcal scalded skin syn-
Skin and Soft Tissue Infections drome (SSSS).
l They include folliculitis, furuncles (boil), styes, boils, l The severe form of SSSS is known as Ritter’s disease in
abscesses, carbuncles, impetigo and pemphigus neona- the newborn and toxic epidermal necrolysis in older
torum. patients.
l Sepsis in wounds and burns. l Milder forms are bullous impetigo and pemphigus neo-
natorum.
Deep Infections
Toxic Shock Syndrome
1. Musculoskeletal infections
a. Acute osteomyelitis Toxic shock syndrome is a potentially fatal multisystem
b. Tropical pyomyositis disease produced by S. aureus strains usually presenting
c. Arthritis and bursitis with fever, hypotension, myalgia, vomiting, diarrhoea, mu-
2. Respiratory infections: In the respiratory tract, it causes cosal hyperaemia and an erythematous rash which desqua-
tonsillitis, pharyngitis, sinusitis, otitis, bronchopneumo- mates subsequently. This is associated with infection of
nia, lung abscess, empyema and rarely pneumonia. mucosal or sequestrated sites by the toxic shock syndrome
3. Central nervous system: Abscess, meningitis, and intra- toxin (TSST) producing S. aureus.
cranial thrombophlebitis
Q. 2. Describe Staphylococcus aureus.
4. Endovascular infections: Bacteraemia, septicaemia, pyae-
mia, and endocarditis Ans.
5. Urinary infections: Staphylococci do cause urinary tract
infections in association with local instrumentation, MORPHOLOGY OF STAPHYLOCOCCUS
implants or diabetes. AUREUS
l They are spherical cocci, approximately 1 mm in diam-
Staphylococcal Food Poisoning eter, arranged characteristically in grape-like clusters.
l They may be found singly, in pairs and in short chains
l Staphylococcal food poisoning results on consumption of three or four cells. Long chains never occur.
of food contaminated with enterotoxin producing staph- l They are Gram-positive, nonsporing, nonmotile, non-
ylococci. capsulated, aerobic and normally facultative anaerobic.
l Sufficient time about 6 h should elapse between the
contamination of the food and ingestion of the contami-
nated food so that the organisms can produce enough
CULTURAL CHARACTERISTICS
enterotoxin. Ingestion of enterotoxin as little as micro- l On ordinary media, they grow readily at an optimum
gram amounts can cause illness like vomiting and diar- temperature of 37°C and pH 7.4–7.6.
rhoea in humans. l On nutrient agar, S. aureus produces large around 2–4 mm
l The emetic effect is brought about by the stimulation of diameter, circular, convex, smooth, raised, shiny, opaque
the CNS through the neural receptors in the GIT. and easily emulsifiable colonies. Though some of them
l The toxin also exhibits pyrogenic, mitogenic, hypoten- may be white, orange or yellow, most strains produce
sive, thrombocytopenic and cytotoxic effects. golden yellow pigment.
288 Quick Review Series: BDS 2nd Year
Part II
Systemic Bacteriology
Topic 16
Staphylococcus, Streptococcus, Pneumococcus
and Neisseria
LONG ESSAYS
Q. 1. Describe the morphology, staining characters and 5. MacConkey’s medium: Due to fermentation of lactose
pathogenicity of Staphylococcus. Add a note on labora- the colonies on the MacConkey’s medium are pink in
tory diagnosis of staphylococcal infections. colour and are very small about pinhead size.
6. Liquid medium: In liquid medium uniform turbidity is
Ans.
produced.
MORPHOLOGY OF STAPHYLOCOCCUS 7. Selective media: Several selective media are used for
isolating S. aureus from specimens such as faeces. These
AUREUS include following media:
1. They are spherical cocci, approximately 1 mm in diam- a. Salt milk agar or salt broth (media containing NaCl)
eter, arranged characteristically in grape-like clusters b. Ludlam’s medium (media containing lithium chlo-
due to cell division occurring in three planes, with ride and tellurite)
daughter cells tending to remain in close proximity. c. Media containing polymyxin
2. They may be found singly, in pairs and in short chains
of three or four cells. Long chains never occur.
PATHOGENICITY
3. They are Gram-positive, nonsporing, nonmotile, non-
capsulated, aerobic and normally facultative anaerobic. 1. Staphylococcus produces two types of diseases as follows:
a. Infections
b. Intoxications
CULTURAL CHARACTERISTICS 2. In the infections cocci gain access to damaged skin,
1. They grow readily on ordinary media within a tempera- mucosal or tissue sites colonized by adhering to cells or
ture range of 10–42°C, the optimum temperature being extracellular matrix, evade host defence mechanisms
37°C and pH 7.4–7.6. multiply and cause tissue damage.
2. On nutrient agar after incubation for 24 h, S. aureus pro- 3. In intoxications the disease is caused by the bacterial toxins
duces large around 2–4 mm diameter, circular, convex, produced either in the infected host or preformed in vitro.
smooth, raised, shiny, opaque and easily emulsifiable
colonies.
STAPHYLOCOCCAL DISEASES
a. Though some of them may be white, orange or yel-
low, most strains produce golden yellow pigment. S. aureus is an important pyogenic organism and its lesions
b. The pigment production occurs optimally at 22°C are characteristically localized in nature.
and only in aerobic cultures and it can be enhanced Common staphylococcal infections are as follows:
by incorporation of 1% glycerol monoacetate or
milk in the medium.
A. Skin and Soft Tissue Infections
3. Nutrient agar slope: The growth on nutrient agar slope is
confluent presenting a characteristic oil paint appearance. 1. These may be of varying severity and are important
4. Blood agar: Colonies of S. aureus on blood agar are cause of hospital acquired infection.
similar to those on the nutrient agar. Most strains are 2. They include folliculitis, furuncles (boil), styes, boils, ab-
haemolytic especially when incubated under 20–25% scesses, carbuncles, impetigo and pemphigus neonatorum.
CO2 and they produce marked haemolysis on rabbit or 3. Sepsis in wounds and burns is also usually caused by
sheep blood and weak on horse blood agar. the S. aureus.
,Click here to Visit - www.thedentalhub.org.in
Section | III Microbiology 289
B. Deep Infections 2. The underlying pathogenic mechanism is the stripping
of the superficial layers of the skin from the underlying
i. Musculoskeletal infections
tissues by the exfoliative or epidermolytic toxin.
a. Acute osteomyelitis is the commonest deep infection,
3. The severe form of SSSS is known as Ritter’s disease in the
the bacteria being implicated as the cause in 90% cases.
newborn and toxic epidermal necrolysis in older patients.
b. Tropical pyomyositis is formation of multiple ab-
4. Milder forms are bullous impetigo and pemphigus neo-
scesses in the voluntary muscles observed usually in
natorum.
young adults.
c. Arthritis and bursitis
ii. Respiratory infections: In the respiratory tract, it causes E. Toxic Shock Syndrome
tonsillitis, pharyngitis, sinusitis, otitis, bronchopneumo- Toxic shock syndrome is a potentially fatal multisystem
nia, lung abscess, empyema and rarely pneumonia. disease produced by S. aureus strains usually presenting
iii. Central nervous system: Abscess, meningitis, intracra- with fever, hypotension, myalgia, vomiting, diarrhoea, mu-
nial thrombophlebitis. cosal hyperaemia and an erythematous rash which desqua-
iv. Endovascular infections: Bacteraemia, septicaemia, py- mates subsequently. This is associated with infection of
aemia, and endocarditis. Staphylococcal septicaemia is mucosal or sequestrated sites by the toxic shock syndrome
rare but a serious condition. toxin (TSST) producing S. aureus.
v. Urinary infections: Staphylococci are uncommon in 1. Staphylococcal enterotoxins and toxic shock syndrome
routine urinary tract infections, though they do cause toxin-1 (TSST-1) are superantigens and potent activa-
infections in association with local instrumentation, tors of T lymphocytes.
implants or diabetes. Renal abscess may be produced 2. They stimulate very large number of T cells without rela-
due to haematogenous spread. tion to their epitope specificity. This leads to an excessive
and dysregulated immune response with release of cyto-
C. Staphylococcal Food Poisoning kines IL-1 and IL-2, tumour necrosis factor, and interferon-
gamma. This is responsible for multisystem involvement.
1. Staphylococcal food poisoning results on consumption 3. It was first observed in 1978 in children and adolescents
of food contaminated with enterotoxin producing staph- but showed wide outbreaks in 1980 in menstruating
ylococci. women using highly absorbent vaginal tampons.
2. The bacteria produce this toxin when grown in carbohy- 4. Convalescents show TSST–1 antibody is protective and
drate and protein foods. Common food items usually absence of it is a factor in the pathogenesis of the condition.
involved are meat, fish, milk and milk products.
3. Sufficient time should elapse between the contamina-
LABORATORY DIAGNOSIS
tion of the food and ingestion of the contaminated food
so that the organism can produce enough enterotoxin. Specimen to be collected depends on type of lesion as
The diarrhoea and vomiting sets in within 6 h of ingest- follows:
ing the contaminated food. 1. Pus from suppurative lesions
a. Enterotoxin is a heat-stable toxin and is also resistant 2. Sputum for respiratory infections
to action of GIT enzymes. 3. CSF for meningitis
b. Eight antigenic types of enterotoxins are currently 4. Blood for septicaemia
known, named A, B, C1–3, D, E and H of which type A 5. Suspected food, vomit or feces for food poisoning
is responsible for most of the cases.
Diagnosis is made by the following:
c. The toxin is believed to act directly on autonomic
nervous system to cause the illness, rather than on GI
a. Direct microscopy
mucosa.
d. Ingestion of enterotoxin as little as microgram amounts 1. Examination of a Gram-stained smear of pus or other
can cause illness like vomiting and diarrhoea in humans. specimen may show Gram-positive cocci in clusters
The emetic effect is brought about by the stimulation of with some single and paired cocci.
the CNS through the neural receptors in the GIT. 2. This is useful for pus and not for other specimen such
e. The toxin also exhibits pyrogenic, mitogenic, hypo- as sputum or feces where mixed bacterial flora is nor-
tensive, thrombocytopenic, and cytotoxic effects. mally present.
b. Culture
D. Exfoliative Diseases
Diagnosis is readily made by culture.
1. Exfoliative toxin is also known as ET or exfoliatin and
is responsible for the staphylococcal scalded skin syn- 1. The specimen is plated on the nutrient agar or blood
drome (SSSS). agar and incubated overnight. S. aureus colonies are
,Click here to Visit - www.thedentalhub.org.in
290 Quick Review Series: BDS 2nd Year
identified as large, circular, convex, smooth, raised, 2. It is exacting in nutritive requirements and growth oc-
shiny and opaque colonies on both the media. curs only in media enriched with blood or serum or
2. Specimens such as feces for food poisoning, swabs media containing fermentable carbohydrates.
from carriers where the bacteria are scanty, are inocu- 3. On blood agar, after incubation the colonies are small,
lated on selective media like Ludlam’s or salt-milk agar circular, semitransparent, convex discs with beta hae-
or Robertson’s cooked meat medium containing 10% molysis.
NaCl. 4. Growth and haemolysis are promoted by 10% CO2.
3. The smears from the culture media are examined under Virulent strains on fresh isolation from lesions produce
the microscope and biochemical tests done to confirm a matt or finely granular colonies while avirulent strains
the S. aureus. form glossy colonies. Strains with well-marked cap-
sules form mucoid colonies.
c. Coagulase test
PATHOGENESIS
1. Coagulase test is done by two methods as follows:
a. Tube coagulase test 1. Streptococcus pyogenes adheres to the pharyngeal epi-
b. Slide coagulase test thelium by means of lipoteichoic acid covering the sur-
2. Tube coagulase test detects free coagulase. The slide face epithelial cells.
coagulase test detects bound coagulase and usually 2. The infection may spread to the surrounding tissues
gives results parallel with tube test. leading to suppurative complications like otitis media,
3. When there is a divergence, the tube test will be decid- mastoiditis, quinsy, Ludwig’s angina and suppurative
ing factor. adenitis. Meningitis may occur rarely.
3. Scarlet fever occurs when the infecting strain of S. pyo-
genes produces erythrogenic toxin in a patient usually a
d. Antibiotic sensitivity tests child with no antitoxic immunity leading to a combina-
1. Should be done simultaneously as a guide to treatment. tion of sore throat and a generalized erythematous rash.
This is important as staphylococci readily develop resis-
tance to drugs.
LABORATORY DIAGNOSIS
Specimens collected are pus, throat swab, sputum, genital
e. Serological tests swabs, blood and CSF. Specimens can be transported in
1. These may sometimes help to diagnose hidden deep Pike’s medium.
infections. 1. Microscopy: The presence of Gram-positive cocci in chains
2. ASO titre more than 2 units/mL with rising titre is of under Gram’s staining is indicative of Streptococcus
value in diagnosing deep seated infections such as bone infections. But smears from throat and genitalia are of no
abscess. value, where Streptococcus forms part of normal flora.
2. Culture: The specimen is plated on blood agar and in-
Q. 2. Write in detail about the morphology, pathogene- cubated at 37°C anaerobically or under 5–10% CO2.
sis, laboratory diagnosis and the treatment of Strepto- The growth is identified by beta haemolysis, Gram’s
coccus pyogenes. staining and antibiotic sensitivity. S. pyogenes is more
Ans. sensitive to bacitracin, hence, a filter paper disc dipped
in a solution of bacitracin (1 unit/mL) is applied on the
MORPHOLOGY OF STREPTOCOCCUS surface of an inoculated blood agar. After incubation,
1. They are Gram-positive cocci arranged in chains or wide zone of inhibition is seen with S. pyogenes, but not
pairs. Individual cocci are spherical or oval in shape, with other streptococci.
measuring 0.5–1.0 mm in diameter. 3. Serology
2. Chain formation is due to cocci dividing in one plane a. Haemolytic streptococci are grouped by Lancefield’s
only and daughter cells failing to separate completely. method. The fluorescent antibody technique has
3. They are nonmotile and nonsporing. Some strains of S. pyo- been employed for rapid identification of group A
genes and some group C strains possess capsules made of streptococci.
hyaluronic acid while polysaccharide capsules are found in b. In poststreptococcal lesions ASO test is used. ASO
groups B and D. Capsules are best seen in young cultures. test is used for retrospective diagnosis of rheumatic
fever and glomerulonephritis.
c. Streptolysin O is antigenic and antistreptolysin ap-
Cultural Characteristics pears in sera following streptococcal infections. Es-
1. It is an aerobe and facultative anaerobe growing best at timation of this antibody known as antistreptolysin,
the temperature of 37°C and an optimum pH of 7.2–7.4. is used as a standard serological procedure for
, Click here to Visit - www.thedentalhub.org.in
Section | III Microbiology 291
retrospective diagnosis of poststreptococcal compli- a convenient, specific, sensitive screening test. It becomes
cations. positive after all types of streptococcal infections.
d. ASO titres higher than 200 are indicative of poststrep-
tococcal infection. High levels are found in acute
TREATMENT
rheumatic fever, but in glomerulonephritis, titres are
low. 1. All beta haemolytic group A streptococci are sensitive
e. DNAase B test is also used. Titres of higher than 300 to penicillin G and most are sensitive to erythromycin.
are taken as significant. 2. In patients allergic to pencillin, erythromycin or cepha-
4. Streptozyme test lexin may be used.
A passive slide haemagglutination test using RBC sen- 3. Antimicrobial drugs have no effect on established glo-
sitized with crude extracellular antigens of Streptococcus is merulonephritis and rheumatic fever.
SHORT ESSAYS
Q. 1. Lesions caused by Staphylococcus aureus Exfoliative Diseases
Ans. Common staphylococcal infections are as follows: l Exfoliative toxin is also known as ET or exfoliatin and
is responsible for the staphylococcal scalded skin syn-
Skin and Soft Tissue Infections drome (SSSS).
l They include folliculitis, furuncles (boil), styes, boils, l The severe form of SSSS is known as Ritter’s disease in
abscesses, carbuncles, impetigo and pemphigus neona- the newborn and toxic epidermal necrolysis in older
torum. patients.
l Sepsis in wounds and burns. l Milder forms are bullous impetigo and pemphigus neo-
natorum.
Deep Infections
Toxic Shock Syndrome
1. Musculoskeletal infections
a. Acute osteomyelitis Toxic shock syndrome is a potentially fatal multisystem
b. Tropical pyomyositis disease produced by S. aureus strains usually presenting
c. Arthritis and bursitis with fever, hypotension, myalgia, vomiting, diarrhoea, mu-
2. Respiratory infections: In the respiratory tract, it causes cosal hyperaemia and an erythematous rash which desqua-
tonsillitis, pharyngitis, sinusitis, otitis, bronchopneumo- mates subsequently. This is associated with infection of
nia, lung abscess, empyema and rarely pneumonia. mucosal or sequestrated sites by the toxic shock syndrome
3. Central nervous system: Abscess, meningitis, and intra- toxin (TSST) producing S. aureus.
cranial thrombophlebitis
Q. 2. Describe Staphylococcus aureus.
4. Endovascular infections: Bacteraemia, septicaemia, pyae-
mia, and endocarditis Ans.
5. Urinary infections: Staphylococci do cause urinary tract
infections in association with local instrumentation, MORPHOLOGY OF STAPHYLOCOCCUS
implants or diabetes. AUREUS
l They are spherical cocci, approximately 1 mm in diam-
Staphylococcal Food Poisoning eter, arranged characteristically in grape-like clusters.
l They may be found singly, in pairs and in short chains
l Staphylococcal food poisoning results on consumption of three or four cells. Long chains never occur.
of food contaminated with enterotoxin producing staph- l They are Gram-positive, nonsporing, nonmotile, non-
ylococci. capsulated, aerobic and normally facultative anaerobic.
l Sufficient time about 6 h should elapse between the
contamination of the food and ingestion of the contami-
nated food so that the organisms can produce enough
CULTURAL CHARACTERISTICS
enterotoxin. Ingestion of enterotoxin as little as micro- l On ordinary media, they grow readily at an optimum
gram amounts can cause illness like vomiting and diar- temperature of 37°C and pH 7.4–7.6.
rhoea in humans. l On nutrient agar, S. aureus produces large around 2–4 mm
l The emetic effect is brought about by the stimulation of diameter, circular, convex, smooth, raised, shiny, opaque
the CNS through the neural receptors in the GIT. and easily emulsifiable colonies. Though some of them
l The toxin also exhibits pyrogenic, mitogenic, hypoten- may be white, orange or yellow, most strains produce
sive, thrombocytopenic and cytotoxic effects. golden yellow pigment.
