Altogether, MacConkey agar only grows gram-negative bacteria, and those bacteria will appear differently based on their lactose fermenting ability as well as the rate of fermentation and the presence of a capsule or not.

E. coli is described as a Gram-negative bacterium. This is because they stain negative using the Gram stain. The Gram stain is a differential technique that is commonly used for the purposes of classifying bacteria.

Selective and Differential Media MacConkey agar not only selects for Gram-negative organisms by inhibiting Gram-positive organisms and yeast but also differentiates the Gram-negative organisms by lactose fermentation. Escherichia coli and other lactose ferments will produce yellow or orange colonies.

Sorbitol MacConkey agar is a variant of traditional MacConkey agar used in the detection of E. coli O157:H7. This is important because gut bacteria, such as Escherichia coli, can typically ferment lactose, while important gut pathogens, such as Salmonella enterica and most shigellas are unable to ferment lactose.

MacConkey’s Agar Is Selective MacConkey’s is a selective medium that inhibits the growth of Gram-positive bacteria due to the presence of crystal violet and bile salts. Gram-negative bacteria grow well on MAC.

Result Interpretation on MacConkey Agar Lactose fermenting strains grow as red or pink and may be surrounded by a zone of acid precipitated bile. The red colour is due to production of acid from lactose, absorption of neutral red and a subsequent colour change of the dye when the pH of medium falls below 6.8.

Streptococcus pneumoniae (S. Hence, it does not grow on MacConkey agar (MA) due to the absence of blood as well as due to the presence of high concentration of bile in the medium which is inhibitory to the growth of S. pneumoniae.

MacConkey agar selects for organisms like Escherichia coli (Gram negative bacilli) while inhibiting the growth of organisms like Staphylococcus aureus (Gram positive cocci). The plate on the right has Staphylococcus aureus growing and changes the color of the agar to yellow.

Staph. aureus will grow on general culture media such as Blood Agar and chocolated Blood Agar and therefore can be isolated from direct plating of clinical specimens. More specialised media, such as Staph/Strep Selective Medium contain antimicrobials.

MacConkey Agar is Selective for Non-fastidious Gram-negative organisms. Therefore, MacConkey needed a way to limit this background of environmental flora and allow only his organisms of interest to grow. A medium that can perform this function is now known as a selective medium.

MacConkey Agar without Crystal Violet The lack of crystal violet permits the growth of Staphylococcus and Enterococcus. Staphylococci produce pale pink to red colonies and enterococci produce compact tiny red colonies either on or beneath the surface of the medium.

Blood Agar is used to grow a wide range of pathogens particularly those that are more difficult to grow such as Haemophilus influenzae, Streptococcus pneumoniae and Neisseria species. It is also required to detect and differentiate haemolytic bacteria, especially Streptococcus species.

MacConkey Agar is a selective medium, Is it also a defined or an undefined medium? Why is that formulation desirable? Undefined, it’s beneficial because it gives a wide variety of growth ability. Only gram-negatives will grow on MacConkey, so you always want to streak to something that you know it will grow on.

Diagnosis is based on performing tests with colonies. Tests for clumping factor, coagulase, hemolysins and thermostable deoxyribonuclease are routinely used to identify S aureus. Commercial latex agglutination tests are available. Identification of S epidermidis is confirmed by commercial biotyping kits.

Staphylococcus aureus or “staph” is a type of bacteria found on human skin, in the nose, armpit, groin, and other areas. While these germs don’t always cause harm, they can make you sick under the right circumstances.

Staphylococcus aureus is Gram-positive bacteria (stain purple by Gram stain) that are cocci-shaped and tend to be arranged in clusters that are described as “grape-like.” On media, these organisms can grow in up to 10% salt, and colonies are often golden or yellow (aureus means golden or yellow).

Staphylococci are gram-positive aerobic organisms. Staphylococcus aureus is the most pathogenic; it typically causes skin infections and sometimes pneumonia, endocarditis, and osteomyelitis.

It is the leading cause of skin and soft tissue infections such as abscesses (boils), furuncles, and cellulitis. Although most staph infections are not serious, S. aureus can cause serious infections such as bloodstream infections, pneumonia, or bone and joint infections.

The genus Staphylococcus currently contains 35 species. Staphylococci are Gram-positive cocci that grow in clumps, are catalase test positive and coagulase test positive (Staph. aureus) or negative (coagulase-negative staphylococci). Streptococci are Gram-positive cocci that grow in pairs or chains.

What are Staphylococcal (staph) infections? Staphylococcus (staph) is a group of bacteria. There are more than 30 types.

The most common type of staph infection is the boil, a pocket of pus that develops in a hair follicle or oil gland. The skin over the infected area usually becomes red and swollen. If a boil breaks open, it will probably drain pus. Boils occur most often under the arms or around the groin or buttocks.