Finchfetch Microorganisms -

Culture Medias

1 Baird-Parker Agar (BPA) his is a selective medium for the isolation of Staphylococcus species. It has lithium chloride and glycine to inhibit the growth of Gram-negative bacteria and most Gram-positive bacteria except Staphylococcus. Egg yolk emulsion is added to detect lecithinase production and tellurite reduction. S. aureus colonies on BPA are black due to reduction of tellurite, and they exhibit a clear zone due to lecithinase activity on egg yolk.

2 Phenol Red Mannitol Broth This is a differential medium, used to determine an organism's ability to ferment mannitol. S. aureus, which can ferment mannitol, will change the medium from red to yellow due to acid production.

3 DNase Test Agar S. aureus produces the enzyme DNase which hydrolyses DNA. When S. aureus grows on this medium, the DNA is broken down, which can be visualised using a hydrochloric acid (HCl) solution: clear zones around the colonies indicate DNA breakdown.

4 Mannitol Salt Agar (MSA), or Chapman Agar This is a selective and differential medium. The high concentration of NaCl (~7.5%) selects for halophiles, organisms that can tolerate high salt concentrations, thereby favouring the growth of Staphylococcus species. Mannitol is the differential component: S. aureus ferments mannitol, lowering the pH of the medium, which results in a color change of the pH indicator from red to yellow. Thus, growth of S. aureus is indicated by yellow colonies.

5 Blood Agar (BA) This is a nutrient-rich, differential medium that supports the growth of many organisms. S. aureus forms colonies that are round, smooth, and golden-yellow. This bacterium typically demonstrates β-hemolysis, which is complete lysis of red blood cells, resulting in a clear zone around the colonies. This hemolysis is due to the production of hemolysins by S. aureus.

About

Staphylococcus aureus is a Gram-positive, round bacterium found in clusters. It is a common source of post-injury or surgical infections, affecting approximately 500,000 patients in American hospitals annually. This bacterium is present in all mammalian species and can transmit between species. Around 30 pourcent of healthy humans carry S. aureus in their nose, throat, and skin. Though S. aureus is carried by one-third of healthy individuals, it generally does not cause disease unless there is a break in the skin or other entry sites. It is considered an opportunistic bacterium, causing infections when the opportunity arises, particularly in immunocompromised or immunodeficient individuals. S. aureus can cause a variety of conditions, from minor skin infections to life-threatening diseases such as pneumonia, meningitis, osteomyelitis, endocarditis, toxic shock syndrome, bacteremia, and sepsis. Diagnosis can be challenging due to the bacterias prevalence on the skin, nose, and throat of humans and animals. However, it can be identified via culture of suspicious lesions, presenting as characteristic yellow to white colonies on blood agar.

Characteristics

1/ Gram stain: Staphylococci appear as Gram-positive cocci that occur singly and in pairs, tetrads, short chains, and irregular grape-like clusters, 2/ Catalase Test: Positive, 3/ Coagulase Test: Positive, 4/ Non-motile, 5/ Non-sporing, 6/ Often unencapsulated or have a limited capsule, 7/ Facultative anaerobes.

Manifestation

Staphylococcus aureus, often shortened to S. aureus, can lead to a number of health problems. These include: 1/ Skin Infections & Surgical wound infections, 2/ Osteomyelitis, 3/ Food poisoning/gastroenteritis, 4/ Toxic shock syndrome, 5/ Pneumonia (mainly hospital-acquired), 6/ Acute endocarditis, 7/ Infective arthritis, 8/ Necrotizing fasciitis, 9/ Sepsis and Staphylococcal scalded skin syndrome (SSSS)

Laboratory Diagnosis

The laboratory diagnosis of a Staphylococcus aureus infection includes: 1/ Gram staining: Gram-positive cocci in clusters, cocci may appear singly in pairs or in short chains. 2/ Sample Collection: Depending on the suspected infection site, a sample (like a swab or blood) is collected. 3/ Culture and Identification: - Blood Agar: The sample is cultured on blood agar. After incubation for 18 to 24 hours, S. aureus typically forms round, golden-yellow colonies with or without beta hemolysis. - Mannitol Salt Agar (MSA): This selective medium is also used for the isolation of S. aureus. After inoculation, MSA plates are incubated at 35°C for 24 to 48 hours. S. aureus, being a mannitol fermenting bacterium, forms yellow or gold colonies on MSA.4/ Gram Stain and Catalase Test: S. aureus is Gram-positive (stains purple) and catalase-positive (produces bubbles when mixed with hydrogen peroxide). 5/ Coagulase Test: S. aureus produces an enzyme called coagulase that causes blood plasma to clot. 6/ Antibiotic Susceptibility Testing: This test identifies which antibiotics the bacteria are sensitive to, guiding treatment. 7/ Molecular Testing: Techniques like PCR may be used for further identification or to detect virulence factors or antibiotic resistance genes.