Staphylococcus: Morphology, Cultural Characteristics, Pathogenicity, Antibiotic Sensitivity
Staphylococcus is a genus of Gram-positive bacteria known for their spherical or grape-like clustered shape under a microscope. They are commonly found in various environments, including on the skin and mucous membranes of humans and animals. Staphylococcus bacteria can be either beneficial or pathogenic, with some species causing infections in humans. Important pathogenic species are:
- Staphylococcus aureus
- Staphylococcus epidermidis
- Staphylococcus saprophyticus
The morphology of Staphylococcus aureus, like all bacteria, can be described in terms of its shape, arrangement, and other structural features. Here is a description of the morphology of Staphylococcus aureus:
Staphylococcus aureus is a spherical bacterium, commonly referred to as a coccus. The cocci can appear as individual cells or in clusters.
Staphylococcus bacteria are known for their characteristic arrangement in grape-like clusters. This cluster formation is due to the irregular and random division of cells in multiple planes, resulting in a three-dimensional, densely packed arrangement.
The individual cells of Staphylococcus aureus are typically around 0.5 to 1.0 micrometers (µm) in diameter. The cluster size can vary but is usually larger and can be seen without a microscope.
Staphylococcus aureus is classified as a Gram-positive bacterium. This means that it retains the crystal violet stain during the Gram staining process, indicating that it has a thick layer of peptidoglycan in its cell wall.
Staphylococcus aureus has a thick, peptidoglycan-rich cell wall, which contributes to its Gram-positive staining. The peptidoglycan layer provides structural support to the cell and plays a crucial role in maintaining cell shape and integrity.
Some strains of Staphylococcus aureus may produce a polysaccharide capsule that surrounds the cell wall. The capsule can protect the bacterium from host immune defenses and is associated with increased virulence.
Staphylococcus aureus is generally non-motile, meaning it does not possess flagella or other structures for active movement.
Staphylococcus aureus does not form endospores. Endospores are highly resistant, dormant structures formed by some bacteria for survival in harsh conditions.
Staphylococcus aureus, like many bacteria, can exhibit specific cultural characteristics when grown in a laboratory setting. These characteristics can help in its identification and differentiation from other bacteria. Here are some key cultural characteristics of Staphylococcus aureus:
Staphylococcus aureus can grow on a variety of solid and liquid media. Commonly used solid media for culturing S. aureus include nutrient agar, blood agar, and Mannitol Salt Agar (MSA). Blood agar, in particular, is often used because it allows for the detection of hemolysis, which is important for differentiation.
Colonies of Staphylococcus aureus on solid agar are typically round, smooth, and slightly raised. The color can vary from white or cream to golden yellow, depending on the strain.
S. aureus can exhibit different types of hemolysis on blood agar, which is an important diagnostic feature. These include:
- Beta-hemolysis: Clear zones around colonies, indicating complete lysis of red blood cells.
- Alpha-hemolysis: Greenish discoloration around colonies, indicating partial lysis of red blood cells.
- Gamma-hemolysis: No hemolysis, indicating no lysis of red blood cells.
Staphylococcus aureus is positive for coagulase production. This test involves adding the bacterial isolate to plasma or a specific reagent and checking for the formation of a clot. A positive result indicates the presence of coagulase, a key characteristic of S. aureus.
S. aureus can ferment mannitol, a sugar, to produce acid. On Mannitol Salt Agar (MSA), colonies of S. aureus turn the phenol red indicator from red to yellow, indicating acid production. This is a distinguishing characteristic as some other staphylococci cannot ferment mannitol.
Staphylococcus aureus is positive for catalase production. This test involves adding hydrogen peroxide to a bacterial colony, and the presence of bubbles indicates the release of oxygen gas, confirming catalase production.
S. aureus is Gram-positive, and this characteristic is determined through a simple Gram stain procedure. The bacteria will appear purple when stained.
Staphylococcus aureus is mesophilic, meaning it grows optimally at temperatures around 37°C (98.6°F), which is close to human body temperature.
S. aureus is relatively salt-tolerant and can grow on media with high salt concentrations, such as Mannitol Salt Agar (MSA), which contains 7.5% salt. The ability to grow on such media is a distinguishing feature.
ENZYMES OF STAPHYLOCOCCUS AUREUS
Staphylococcus aureus, like many pathogenic bacteria, produces a variety of enzymes that contribute to its virulence and ability to cause infections. Some of the important enzymes produced by Staphylococcus aureus include:
Staphylococcus aureus produces two types of coagulases, bound coagulase and free coagulase. These enzymes promote the coagulation of blood plasma, leading to the formation of fibrin clots. The ability of S. aureus to coagulate blood is a diagnostic feature used in laboratory tests to identify the bacterium.
S. aureus produces catalase, an enzyme that breaks down hydrogen peroxide (H2O2) into water and oxygen. This ability to neutralize hydrogen peroxide helps the bacterium evade host immune defenses, as it can survive in the presence of reactive oxygen species produced by host cells.
Hyaluronidase is an enzyme that degrades hyaluronic acid, a component of connective tissue. This enzyme aids in the spread of the bacterium through host tissues, allowing it to invade and cause deeper-seated infections.
Staphylokinase is an enzyme that dissolves fibrin clots, which can help the bacterium evade the host’s immune response by breaking down blood clots that form around the site of infection.
Staphylococcus aureus produces lipase, which can hydrolyze lipids and fats. This enzyme may contribute to the degradation of lipids in sebaceous secretions on the skin and can facilitate the colonization of skin and mucous membranes.
S. aureus produces a DNA nuclease that can break down host DNA. This enzyme can help the bacterium evade the host immune system by degrading extracellular DNA traps formed by neutrophils to entrap pathogens.
Staphylococcus aureus produces various proteases, including aureolysin and V8 protease. These enzymes can degrade host proteins and contribute to tissue damage and invasion.
Fibrinolysin is an enzyme that dissolves fibrin clots, similar to staphylokinase. It can help S. aureus spread within the host.
9. DNase (Deoxyribonuclease):
S. aureus produces DNase enzymes that degrade DNA. This can aid in immune evasion by breaking down extracellular DNA traps, which are part of the host defense mechanism.
TOXINS OF STAPHYLOCOCCUS AUREUS
Staphylococcus bacteria, especially Staphylococcus aureus, produce a variety of toxins that contribute to their pathogenicity. These toxins can cause a range of symptoms and diseases when they infect the host. Some of the most important Staphylococcus toxins include:
1. Alpha-Toxin (α-toxin):
This cytolytic toxin damages cell membranes and can cause the lysis (bursting) of various host cells, such as red and white blood cells, as well as other cells at the site of infection. It can also lead to tissue destruction.
2. Beta-Toxin (β-toxin):
Beta-toxin is a sphingomyelinase that affects cell membranes. It plays a role in the destruction of red blood cells and can contribute to the formation of abscesses.
3. Delta-Toxin (δ-toxin):
Delta-toxin is a small, hydrophobic peptide that forms pores in host cell membranes. It can damage erythrocytes (red blood cells) and contribute to cell lysis.
4. Gamma-Toxin (γ-toxin):
Gamma-toxin is a hemolysin that can damage red blood cells and promote the spread of the infection.
5. Exfoliative Toxins:
Staphylococcus aureus produces exfoliative toxins that cause blistering and skin peeling in conditions like staphylococcal scalded skin syndrome. These toxins disrupt the integrity of the epidermis.
Staphylococcal enterotoxins (SEs) are responsible for food poisoning. When they are produced in contaminated food and ingested, they can lead to symptoms like nausea, vomiting, diarrhea, and abdominal cramps.
7. Toxic Shock Syndrome Toxin (TSST-1):
This toxin is associated with toxic shock syndrome (TSS) and can cause a rapid and severe illness with symptoms like high fever, rash, hypotension, and organ failure.
8. Panton-Valentine Leukocidin (PVL):
PVL is a toxin that targets white blood cells (leukocytes) and can contribute to the severity of skin and soft tissue infections caused by Staphylococcus aureus.
9. Staphylococcal Superantigens:
These toxins stimulate an exaggerated immune response by activating a large number of T-cells. They are responsible for some of the intense inflammation seen in Staphylococcus infections.
Staphylococcus aureus can produce various hemolysins, including alpha-hemolysin, beta-hemolysin, and delta-hemolysin, which cause the lysis of red blood cells.
The pathogenic species is S. aureus. It caused:
- Boils, abscesses furuncles and carbuncles
- Wound infection
- Hospital Infections
- Pneumonia, osteomyelitis, meningitis, endocarditis
- Food Poisoning
- Scalded Skin Syndrome in children
- Toxic Shock Syndrome
S. epidermidis is a normal commensal may cause endocarditis especially in prosthetic valves, ventricular shunts and in drug addicts. This organism is also an important cause for intravascular catheter associated blood stream and other infections particularly in immunocompromised patients. S. saprophyticus causes urinary tract infections in females. Both are coagulase and DNAse negative and can be differentiated by putting up the antimicrobial sensitivity disk of Novobiocin or colistin (Polymyxin). S. saprophyticus is resistant to novobiocin and susceptible to Novobiocin and resistant to colistin.
Biochemical Reactions of Staphylococcus aureus:
The antibiotic sensitivity of Staphylococcus aureus, including its susceptibility or resistance to various antibiotics, can vary among different strains and can change over time due to the emergence of resistant strains. Here’s an overview of the antibiotic sensitivity of Staphylococcus aureus:
Methicillin-Sensitive Staphylococcus aureus (MSSA):
MSSA strains are sensitive to methicillin and other beta-lactam antibiotics, including penicillins and cephalosporins. They are typically treatable with a range of antibiotics, such as nafcillin, oxacillin, and vancomycin.
Methicillin-Resistant Staphylococcus aureus (MRSA):
MRSA strains have developed resistance to methicillin and many other beta-lactam antibiotics. They are often resistant to multiple antibiotics, making them challenging to treat. MRSA strains are typically susceptible to antibiotics like vancomycin, linezolid, daptomycin, and newer agents like ceftaroline. However, there have been cases of vancomycin-resistant MRSA (VRSA) and linezolid-resistant MRSA (LRSA), although these remain relatively rare.
Staphylococcus aureus has shown a remarkable ability to develop resistance to multiple antibiotics, partly due to the acquisition of resistance genes and the presence of mobile genetic elements, such as plasmids. As a result, healthcare-associated MRSA (HA-MRSA) and community-associated MRSA (CA-MRSA) strains have emerged, which may have different resistance profiles.
Vancomycin-Intermediate Staphylococcus aureus (VISA) and Vancomycin-Resistant Staphylococcus aureus (VRSA):
Some strains of Staphylococcus aureus have shown reduced susceptibility to vancomycin, a key antibiotic for treating MRSA. In rare cases, VISA and VRSA strains have been reported, making treatment more challenging.
Testing and Surveillance:
To determine the antibiotic sensitivity of specific Staphylococcus aureus strains, clinical laboratories conduct susceptibility testing using methods like the disk diffusion method, E-test, or automated systems. Regular surveillance of antibiotic resistance patterns is essential to monitor the prevalence of resistant strains and guide treatment decisions.
In addition to beta-lactam and vancomycin resistance, some Staphylococcus aureus strains have developed resistance to other classes of antibiotics, such as macrolides, tetracyclines, and aminoglycosides. This has led to the emergence of multidrug-resistant Staphylococcus aureus strains.