Staphylococci Micromodule Flashcards
Describe the Gram-stain of staphylococci
Gram positive round cocci arranged in grape-like clusters
Describe the clinical significance of the Coagulase Test when applied to Staphylococci
The coagulase test refers to the coagulase enzyme, which converts fibrinogen to fibrin. Certain Staph strains produce this enzyme.
In the test, some of the bacteria are mixed with rabbit blood, which contains fibrinogen. If clumping is observed, the reaction has taken place and so the bacteria is coagulase positive Staph.
If it is negative it is coagulase negative Staph
What is a common infection causes by coagulase negative Staphylococcus?
Staphylococcus saprophyticus is a major cause of urinary tract infections in young women
Describe some infections caused by Staphylococcus aureus
Some strains produce toxins which result in specific diseases.
Infections cause by S. aureus are usually pyogenic
Carriage is often in the anterior of the nose
Infections range from minor skin infections, to severe systemic infections. Some of these include folliculitis ( pustules involving hair follicles), and carbuncle.
S. aureus is the most common cause of Osteomyelitis and septic arthiritis
What are the implications of S. aureus bacteraemia?
Bacteraemia - when bacteria are in the blood, usually a sterile site
Can be asymptomatic, or cause a fever and muscle aches.
When bacteria are in the blood, can cause a systemic inflammatory response syndrome (SIRS), which can result in pyrexia, tachycardia, and a raised or diminished white blood cell count. This can go on to hypotension, organ failure and death. This stage is called Septicaemia
Bacteraemia or Septicaemia can be the result of S. aureus establishing secondary infection at virtually any body site
How do you differentiate between Strep and Staph in the lab?
You can use the appearance of bacteria - both are gram positive, but Staph has a characteristic grape like cluster while strep is usually arranged in chains or diplococci.
In addition, strep does not contain the catalase enzyme, so the catalase test can be used - a positive test, where hydrogen peroxide is converted to oxygen and water, would indicate a staph infection.
What’s the difference between alpha and beta haemolysis
Alpha is only partial digestion of the red blood cells of agar, so creates a green ish tinge
Beta is complete haemolysis, so will result in transparent patches around the colonies
How are Strep pathogens classified?
Lancefield typing and haemolytic properties
Lancefield A and B groups are clinically relevant
Lancefield A are called S. pyogenes
Lancefield B are called S. agalactiae
Both Lancefield A and B groups are beta haemolytic.
Alpha-haemolytic Strepococci are part of the body’s normal flora and only become pathogens when they colonise sites they don’t belong in
What are some diseases caused by beta-haemolytic Lancefield group A strep?
Suppurative and non-suppurative.
These are S. pyogenes bacteria, which are usually suppurative, and cause tonsillitis, and cellulitis. Necrotising fascilitis can develop from cellulitis.
Non-suppurative diseases include scarlet fever and rheumatic fever, which develop if pathogens contain certain VF
What are some diseases caused by beta-haemolytic Lancefield group B strep?
S. agalactiae are found in the commensals of the bowel and vagina.
They can cause post-partum sepsis and neonatal infections
What are the species names of important alpha haemolytic Strep, and what do they cause?
S. angionis - predilection to abscesses in the dental, head and neck region
S.mutans - leading cause of dental caries, forming slime on the tooth
S.mitis - most common bacterial cause of non-hospital acquired endocarditis
S.salivarius - uncommon cause of invasive disease, may also be an uncommon cause of meningitis
S.sanguinis - may antagonize S.mutans role in forming dental caries
S.bovis - flora of GIT, can cause bacteraemia and septicaemia if it gets into the blood
Describe the Gram stain of S.pneumonia, basis of serotyping and important diseases in produces
Highly virulent pathogen
Dipplococci and gram positive. Individual cocci are lancet shaped
Major cause of pneumonia and meningitis, particularly in the immunocompromised.
Also causes bronchitis, pericarditis, sepsis etc
More than 90 serotypes are known
What are two important species of Enterococci, where do they reside and what is their role in disease?
These are both found in human bowel commensal flora - E. faecalis and F. faecium.
They are both gram positive and arranged in short chains.
They express the Lancefield D antigen
Faecalis plays a role in septicaemia, endocarditis, urinary tract infections and meningitis
Faecium can cause meningitis and endocarditis as well
Enterococci are inherently very resistant to antibiotics
