Microorganisms infecting the skin

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Microorganisms infecting the skin

• Dr. Sayeed Ismail Khatib
• Lecturer
• Microbiology Immunology
• Ibn Sina National College for Medical Studies

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Introduction

• Skin
• The skin has the largest surface area of all of
  the body organs and is the most exposed organ.
• It is very effective in providing protection
  against the external environment.
• skin infections are common presentations.
• Normal flora of the skin
• Mainly Gram positive Staphylococci,
  Streptococci, Propionibacterium acnes, etc.

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• Skin infections are often the result of a break
  in the integrity of the skin
• Trauma,
• Wound,
• puncture,
• Accident, etc.

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Pathogens of the skin

• Bacteria
• Genus Staphylococci
• Genus Streptococci
• Genus Clostridium
• Fungi
• Genus Candida
• Parasite
• Ectoparasitic skin infection
• by Sarcoptes scabei, variety hominis

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• STAPHYLOCOCCAL INFECTIONS
• (localized pyogenic lesions)
• Skin and soft tissue
• Folliculitis, Furuncle (boil), Abscess
  (particularly breast abscess), wound infection,
  carbuncle, impetigo, paronchia, less often
  cellulitis
• Musculoskeletal
• Osteomyelitis, arthritis, pyomyositis.
• Respiratory Tonsillitis, lung abscess,
  pharyngitis, etc.
• Central nervous system
• Endovascular
• Urinary

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Identification of GPC - Staphylococci

• Common species
• Staphylococcus aureus
• Staphylococcus epidermidis
• Staphylococcus saprophyticus
• Staphyle in Greek means Bunch of grapes
  kokkos means Berry

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Identification Process
Microscopic morphology Gram Positive Cocci in
clusters
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Growth on Nutrient Agar Plate Growth on
Blood agar Plate
Macroscopic morphology On Nutrient
Agarcircular, entire, convex, Yellow Pigment
On Sterile Blood Agarmay show hemolysis
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IDENTIFICATION PROCESS

• Biochemical Tests
• Catalase
• Coagulase
• Mannitol salt Agar growth
• Novobiocin susceptibility

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• 1. Catalase Test
• Place a drop of 3 H2O2 on glass slide.
• Select colonies to be tested
• Mix colonies in drop of H2O2 (Hydrogen Peroxide)
• H2O2 H2O O2

catalase
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Catalase Test
Bubbles positive catalase present
No bubbles negative no catalase present

• All Staphylococcus species are catalase positive

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Catalase -ve
Catalase ve
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• 2. COAGULASE TEST
• Coagulase converts fibrinogen to fibrin
• Pathogenic strains coat themselves in fibrin to
  evade immune system

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Slide test

• Principle Latex particles coated with fibrinogen
  is used to detect coagulase
• place Test Latex in one circle on test card
• place Control Latex in other circle
• Mix the mass

Control
Test Drop
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Control
Test
Control
Test

• Agglutination in test drop

No agglutination in test drop negative test
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Staphylococcus aureus is coagulase positive If
organism is coagulase negative - Culture on
Salt Mannitol agar Novobiocin susceptibility
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3. Growth on Salt Mannitol agar

• streak plate with organism
• if mannitol is fermented - acid is produced
• Phenol red (indicator) turns yellow in presence
  of acid

incubate 18-24 hrs at 37o C Coagulase-negative
Staphylococi Growth seen, but no color change

• Staphylococcus aureus
• Growth AND color change

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4. Novobiocin Susceptibility
streak a nutrient agar plate for confluent
growth.

• place a novobiocin antibiotic
• disc in the center of the lawn
• incubate 18-24 hrs at 37o C

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Novobiocin Susceptibility
Measure Zone of Inhibition
More than 17 mm Sensitive
Less than 17 mm Resistant
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Test Staph. aureus Staph. epidermidis Staph. saprophyticus
coagulase pos neg neg
Mannitol growth pos pos pos
Mannitol fermentation pos neg pos
Novobiocin susceptibility S S R
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• STREPTOCOCCAL INFECTIONS
• (pyogenic infections with a tendency to spread
  locally, along lymphatic's and through blood
  stream.)
• Skin and soft tissue
• Infections of the wounds or burns, cellulitis,
  Infection of minor abrasions may lead to fatal
  septicemia
• Erysipelas and impetigo
• Other suppurative infections
• Str. pyogenes may cause abscess in internal
  organs such brain lungs, liver, kidney and also
  septicemia and pyemia.
• Other suppurative infections
• Acute rheumatic fever and acute
  glomerulonephritis

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Identification of GPC - Streptococci

• Common species
• Streptococcus pyogenes
• Streptococcus agalactiae
• Streptococcus pneumoniae
• Streptococcus bovis
• Enterococcus faecalis
• Streptos means Twisted or coiled
• and kokkos means Berry

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IDENTIFICATION PROCESS

• Microscopic morphology
• Gram Positive Cocci in chains

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Macroscopic morphology On Nutrient
AgarPunctiform, convex, colourless On
Sterile Blood Agarmay show hemolysis
Alpha
Beta
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Classification by Lancefield grouping

• Lancefield classified serologically into groups
  based on Carbohydrate Antigen present on cell
  wall
• A-V identified (without I J)
• Group AStrep. pyogenes
• (Majority of Hemolytic strep. that produce
  human infection)
• Group BStrep. agalactiae (Neonatal
  meningitis)
• Group D Strep. Bovis Enterococcus faecalis

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• Biochemical Tests
• Catalase
• Optochin sensitivity for Strep. pneumoniae
• 3. Bacitracin sensitivity for Group A
  streptococcus

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1. Catalase

• All Strepococcus and Enterococcus are catalase
  negative.
• Will not show bubbles on addition of H2O2
  (Hydrogen peroxide)

No bubbles negative no catalase present
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• 2. Optochin Sensitivity (for Strep. pneumoniae)
• streak for confluent growth
• place optochin disc on lawn
• incubate 18-24 hrs at 35o C

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• Optochin (an alkaloid) diffuses into agar
• organisms susceptible to Optochin will not grow
• zone of inhibition of 15-30 mm considered
  positive

Optochin Sensitive
Optochin Resistant
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3. Bacitracin sensitivity (for Group A Strep.)

• Group A Streptococci are sensitive to Bacitracin.
• Will show zone of inhibition around bacitracin
  disc.

Bacitracin Sensitive
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Test Strep. pyogenes Strep. agalactiae Strep. pneumoniae Enterococcus feacalis Strep. bovis
Optochin Sensitivity pos neg neg
Bacitracin Sensitivity pos neg neg neg
Hemolysis On BAP Beta Beta Alpha Gamma Gamma
Catalase neg neg neg neg neg
Lancefield group A B D D
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CLOSTRIDIUM SPECIES

• Genus Clostridium consists of Gram positive,
  anaerobic, spore forming bacilli.
• Common Species
• Clostridium perfringens Gas gangrene
• Clostridium botulinum Botulism
• Clostridium tetani Tetanus

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• Gas gangrene
• Clostridium perfringens causes most case of gas
  gangrene, which occurs in lacerated wounds such
  as gun wounds.
• The muscle tissue is oedematous and slowly filled
  with gas, which is felt as crepitus.
• There is a tachycardia and the patient dies in
  shock or from renal or hepatic failure.

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Microscopic morphology

• Clostridium perfringens Clostridium tetani
• Gram Positive bacilli Gram positive bacilli
  with drum stick appearance

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Anaerobic Jar Gas Pack

• Produces anaerobic conditions for strict anaerobe
  
• Anaerobic jar converts Oxygen to water by
  chemical reaction. Water is added to the red
  envelope to generate hydrogen gas (H2) and carbon
  dioxide.
• The resulting hydrogen combines at the palladium
  catalyst in the jar lid to convert the oxygen
  present into water, creating the anaerobic
  environment

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Anaerobic Jar Gas Pack
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Fungi Candida albicans

• Candidiasis is by far the most common type of
  yeast infections in human skin.
• Candidiasis is infection with Candida spp.
• Mainly occurring as vaginal infections and oral
  thrust.
• More than 20 species of Candida exist.
• The most common is Candida albicans.

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Candida albicans

• Microscopic Morphology
• Gram Positive ovoid or spherical budding cell
• Produces pseudomycelia

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• Growth on sabourauds agar Blood Agar plate
• forms white mucoid colonies

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Germ tube test

• A rapid method of identification of Candida
  albicans
• Unique ability to produce germ tube in presence
  of serum

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Parasitic infection Scabies

• Infestation is common, found worldwide, and
  affects people of all races and social classes.
• Scabies spreads rapidly under crowded conditions
  where there is frequent skin-to-skin contact
  between people, such as in hospitals,
  institutions, child-care facilities, and nursing
  homes.

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• Ectoparasitic skin infection caused by
• Sarcoptes scabei, variety hominis
• A scabies burrow under magnification. The scaly
  patch at the left is due to scratching of the
  original papule.
• The mite traveled from there to the upper right,
  where it can be seen as a dark spot at the end of
  the burrow.

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• About a month after contact, a rash and intense
  itching appears in the area of the burrows.
• The itching (more pronounced at night) is caused
  by toxic secretions and excretions of the
  burrowing mites.
• Tiny vesicles and papules (blisters) form above
  the burrows on the skin's surface.
• Scratching may cause secondary infections and
  result in suppuration (leakage of plasma) and
  bleeding.