Staphylococcus Quiz on Clinical Relevance

Microbiological Identification of Medically Important Staphylococci

• Staphylococci are Gram-positive bacteria commonly found on human and animal skin and mucous membranes.
• Some species are harmless commensals, while others can cause serious infections in immunocompromised individuals.
• Staphylococcus aureus is a medically significant species known for its virulence factors and ability to cause various diseases.
• Staphylococcus epidermidis and Staphylococcus saprophyticus are coagulase negative species often found as part of the normal flora.

Introduction to Staphylococci

• Staphylococci are ubiquitous, meaning they are present in many environments frequented by humans.
• Clinically relevant species include Staphylococcus aureus and coagulase-negative staphylococci (CoNS). These species can cause serious infections, and some have developed resistance to antibiotics, making treatment challenging.

Staphylococcus aureus

• Morphology: Gram-positive cocci arranged in clusters.
• Cultural characteristics: Facultative anaerobes that grow by aerobic or microaerophilic respiration at 37°C. Colonies are round, smooth, raised, and range in color from grey to golden yellow.

Medically Important Staphylococci

• Staphylococcus aureus: coagulase positive, beta-hemolytic on blood agar; associated with diseases like endocarditis, food poisoning, toxic shock syndrome, and osteomyelitis.
• Staphylococcus epidermidis: coagulase negative, no hemolysis, part of normal flora often associated with infections in medical devices, and related to nosocomial infections.
• Staphylococcus saprophyticus: coagulase negative, no hemolysis, part of normal flora often associated with urinary tract infections in young women.

Asymptomatic Colonization of S. aureus

• A significant percentage of the human population carries S. aureus asymptomatically, often in the nasal passages.
• S. aureus can colonize various moist areas like skin folds, oropharynx, GI tract, and urogenital tract.
• A higher proportion of carriers exists within hospital staff.

Virulence Factors

• Coagulase enzyme: An enzyme-like protein that clots citrated plasma.
• Exfoliative toxin
• Enterotoxins: Cause food poisoning.
• DNase
• Haemolysins The presence of these factors can distinguish medically important strains.

S. aureus Reservoir

• Frequently found in environments frequented by humans.
• Easily isolated from fomites (inanimate objects).
• Healthy adults have a carrier rate of 20-60%, primarily in anterior nares, skin, nasopharynx, and intestine.

Sample Collection and Processing

• The process involves collecting samples from the affected site, such as nasal swabs, skin lesions, or blood.
• Samples are promptly transported to the lab, inoculated onto appropriate culture media, and incubated at 35-37°C for 18-24 hours.
• The presence of distinct colonies characteristic of staphylococci is then observed.

Laboratory Identification of Staphylococcus aureus

• Direct smear with Gram stain showing Gram-positive cocci in clusters within pus cells.
• Culture on blood agar (for hemolytic reactions) and nutrient agar (for pigment production).
• Golden yellow pigment indicates S. aureus.

The Gram Stain

• A critical initial step in identifying staphylococci.
• Staphylococci stain as Gram-positive cocci in clusters.
• Observing the gram-positive and the arrangemnt helps distinguish between staphylococci and other bacteria.

Culture Media and Growth Characteristics

• Staphylococci are typically cultured on nutrient-rich media like blood agar or mannitol salt agar.
• Colonies form with characteristic features such as shape, size and color.
• Many Staphylococcus species produce a golden-yellow carotenoid pigment which aids differentiating them.
• Staphylococci demonstrate various hemolytic reactions, such as alpha or beta hemolysis, on blood agar; this is helpful for identifying them.

Biochemical Tests

• Complete hemolysis on blood agar
• Catalase positive
• Coagulase positive
• Ferments mannitol on mannitol salt agar.

Catalase Test

• A microscopic examination helps confirm the morphology of bacterial cells, specifically confirming the presence of Gram-positive cocci arranged in clusters.
• Obtaining a fresh bacterial colony and mixing it with hydrogen peroxide will demonstrate the presence of bubbles, indicating a positive catalase test for staphylococci.

Coagulase Test

• The coagulase test detects the presence of coagulase enzyme in pathogenic staphylococci.
• A positive result, where plasma clots, indicates the presence of S. aureus.
• A negative result, no clotting of plasma, indicates the presence of less virulent species.

Tube Coagulase Test

• Diluting the plasma and adding the species into tubes.
• Incubating the tubes at 35-37°C
• Observing for clotting at intervals.
• Positive results produce a clot.
• Negative results don't clot (plasma remains liquid.)

Slide Coagulase Test

• A drop of saline is placed on a slide.
• The bacterial colony is emulsified into two thick suspensions.
• A drop of undiluted plasma is added to one suspension and mixed gently.
• Clumping of organisms within 10 seconds indicates a positive result.

DNase Test

• The procedure involves inoculating a bacterial colony onto a DNase test agar plate and incubating it.
• Presence of a clear zone around the colony indicates a positive result, characteristic of Staphylococcus species.

Mannitol Fermentation

• This test differentiates staphylococci based on their ability to ferment mannitol.
• Pathogenic S. aureus ferment mannitol, turning the media yellow while coagulase-negative staphylococci do not.

Mannitol Salt Agar

• Selective for staphylococci due to high sodium chloride concentration.
• Differential due to pH indicator (phenol red) which changes color depending on the fermentation of mannitol.
• A yellow color indicates mannitol was fermented causing an acidic pH.

Antibiotic Susceptibility Testing

• Disk diffusion: A method that determines the susceptibility of a bacterium to an antibiotic by measuring the zones of inhibition around antibiotic disks.
• Automated systems: Rapidly identify bacteria and test their susceptibility to a panel of antibiotics.
• Molecular techniques: Detect antibiotic resistance genes to guide treatment.

S. aureus Diseases

• Infections range from localized (e.g., abscesses, folliculitis) to systemic (e.g., bacteremia, osteomyelitis).
• Toxigenic diseases, such as food poisoning, toxic shock syndrome, and scalded skin syndrome, are also caused by specific types of S. aureus.

Case Studies and Clinical Relevance

• Case studies highlight the clinical manifestations and relevance of Staphylococcus species.
• Examples include osteomyelitis, surgical site infections, and MRSA bacteremia.

Coagulase-Negative Staphylococci

• Staphylococci that test negative for coagulase.
• Staph epidermidis: White pigment on nutrient agar, frequently found in medical devices.
• Staph saprophyticus: Lemon yellow pigment on nutrient agar, associated with urinary tract infections.

Novobiocin Test

-Differentiates between Staph epidermidis and Staph saprophyticus.

• Staph epidermidis is sensitive and Staph saprophyticus is resistant

Micrococcus

• Gram-positive cocci found in tetrads (clusters of four).
• Usually non-pathogenic.
• Distinguishing factor: Catalase-positive.

Methicillin-resistant Staphylococcus aureus (MRSA)

• A variant of Staphylococcus aureus resistant to methicillin and other beta-lactam antibiotics.
• Hospital-associated (HA-MRSA) and community-associated (CA-MRSA) are the two types.
• Vancomycin is often the drug of choice for treating MRSA.

Questions and Answers

A section with summaries of multiple choice questions covering concepts about Staphylococcus species.