Microbiology Quiz: Bacterial Isolation and Antimicrobial Sensitivity

Bacterial Isolation and Identification in Wounds

• 92 swabs were positive for bacterial and fungal isolation, with Pseudomonas aeruginosa being the most common isolate, followed by Staphylococcus aureus, Klebsiella pneumoniae, Escherichia coli, Staphylococcus epidermidis, Streptococcus pyogenes, Proteus mirabilis, and Candida albicans.
• The aim of the research is to clarify the methods of isolation and identification of microorganisms colonized in wounds, burns, and diabetic foot ulcers to help in proper wound management and assist in the control of antibiotic usage.

Infection in Wounds, Burns, and Diabetic Foot Ulcers

• Infection is an important cause of morbidity and mortality in nosocomial wounds, burns, and diabetic foot ulcers, especially in developing countries.
• Exposure of subcutaneous tissue following a loss of skin integrity provides a moist, warm, and enriched environment that is helpful to aerobic and anaerobic microbial colonization and proliferation.
• The diversity and abundance of microorganisms in any wound will be influenced by factors such as wound type, depth, location, and the level of tissue perfusion.

Microorganisms in Diabetic Foot Ulcers

• Staphylococcus aureus is a prevalent isolate in diabetic foot ulcers, together with other bacteria including S. epidermidis, Streptococcus spp., P. aeruginosa, Escherichia coli, and Klebsiella pneumoniae, and fungi, with Candida albicans being the most predominant.
• The polymicrobial nature of diabetic foot ulcers suggests that the treatment of infection could be based on a better understanding of the general microbiology of these wounds.

Bacteria Characteristics

• Bacteria are single-cell organisms that are neither plants nor animals, measuring a few micrometers in length and existing together in communities of millions.
• Bacterial cells are different from plant and animal cells, being prokaryotes, which means they have no nucleus.
• Bacteria can be found in various environments, including soil, water, plants, animals, radioactive waste, deep in the earth’s crust, arctic ice, and glaciers, and hot springs.

Shape and Classification of Bacteria

• Bacteria can be classified into different shapes, including rod-shaped (bacilli), spiral (spirilla), and cocci-shaped.
• Gram-positive bacteria comprise cocci, bacilli, or branching filaments.
• Gram-positive cocci include Staphylococcus (catalase-positive) and Streptococcus (catalase-negative).

Staphylococcus aureus

• Staphylococcus aureus is a gram-positive bacteria that is found in the environment and is also found in normal human flora, located on the skin and mucous membranes of most healthy individuals.
• S. aureus does not normally cause infection on healthy skin, but if it is allowed to enter the bloodstream or internal tissues, it can cause a variety of potentially serious infections.
• S. aureus is a facultative anaerobe, able to grow aerobically or anaerobically, and can grow at temperatures between 18°C and 40°C.

Cultural Characteristics of Staphylococcus aureus

• On Nutrient Agar: Circular, cream-colored to white colonies with an entire margin, raised elevation, and a dense center with transparent borders.
• On Blood Agar: White non-hemolytic colonies.
• On Mannitol Salt Agar: Small pink to red colonies are formed, and the media remains red as the bacterium cannot ferment mannitol.

Staphylococcus epidermidis

• Staphylococcus epidermidis is a gram-positive bacteria that is part of the normal human flora, located on the skin and mucous membranes of most healthy individuals.
• S. epidermidis is a common cause of wound infections, particularly in healthcare settings where the risk of transmission is higher.
• Treatment for S. epidermidis infection largely depends on the type and severity of the infection, and resistance to methicillin is present in more than 80% of the coagulase-negative staph isolates.

Streptococcus

• Streptococcus are gram-positive, catalase-negative, coagulase-negative cocci that occur in pairs or chains.
• Streptococcus bacteria subdivide into Strep. Pyogenes (Group A), Strep. Agalactiae (Group B), enterococci (Group D), Strep. viridans, and Strep. pneumonia.

Proteus mirabilis

• Proteus mirabilis is a gram-negative bacteria that is found abundantly in soil and water, and is part of the normal human intestinal flora.
• P. mirabilis is primarily transmitted through the fecal-oral route and can contaminate food, water, or surfaces touched by infected individuals.
• P. mirabilis can invade wounds through several mechanisms, including swarming motility, extracellular enzymes, urease activity, and biofilm formation.

Cultural Characteristics of Proteus mirabilis

• On Nutrient Agar: Thin, spreading growth (swarm) colonies.
• On Blood Agar: Swarming growth (forms a thin filmy layer of concentric circles) colonies.
• On MacConkey Agar: Pink colonies (lactose fermenter).

Treatment and Prevention of Proteus mirabilis Infection

• To treat P. mirabilis infection, ampicillin, broad-spectrum penicillins, first, second, and third-generation cephalosporins, imipenem, and aztreonam can be used.
• Prevention of P. mirabilis infections can be achieved by proper hygiene, sanitation, wound care, and infection control measures.