Staphylococcus aureus is a Gram-positive bacterium that cause a wide variety of clinical infections. S. aureus is commonly found in the environment and is also a normal component of human flora, residing on the skin and mucous membranes of most healthy individuals [1].

S. aureus infections can vary from superficial skin lesions, through deep seated abscesses to life threating sepsis [2]. Alexander Ogston (1844-1929) was the first to study wound infection in humans, and to him we owe the name staphylococcus [3]. In 1880, Ogston identified staphylococcus aureus as a causative agent of wound infections and abscesses, which was a significant advancement in medical microbiology and surgery. Ogston was particularly interested in the causes of pus formation in wounds. Through his research, he observed grape-like clusters of bacteria in pus samples from infected wounds under the microscope. He named these bacteria “staphylococci” from the Greek word “staphyle,” meaning a bunch of grapes, and “kokkos,” meaning berry. Among these staphylococci, Ogston identified a specific strain that produced a golden-yellow pigment, which he called Staphylococcus aureus. Staphylococcus aureus belongs to the family Staphylococcaceae. It affects all known mammalian species, including humans. Staphylococcus alexandrines, previously known as Staphylococcus ogostoni, is a coagulase-negative staphylococcus that, while less commonly associated with disease compared to Staphylococcus aureus, still holds clinical and microbiological importance. This bacterium is known for its potential to cause infections, particularly in individual with weakened immune systems or those with implanted medical devices making accurate identification and treatment crucial. Its relevance extends to the study of antibiotic resistance, as S. alexandrines can display resistance patterns similar to other coagulase-negative staphylococci, which is important for effective therapeutic strategies. Proper differentiation from other Staphylococcal species is necessary for appropriate clinical management and ongoing research into its epidemiology and pathogenic mechanisms enhances understanding of staphylococcal infections.

Staphylococcus aureus is commonly found on the skin and mucous membranes of humans [4]. It can be considered as a part of the normal or commensal flora in these areas, meaning it typically coexists with the host without causing harm. It is seen commonly in external nose in 20-40% persons.

Staphylococcus aureus can sometimes become pathogenic, especially if it enters the body through cuts, wounds, or other breaches in the skin. When this happens, it can cause various infections, ranging from minor skin infections to more severe conditions like pneumonia, bloodstream infections, or surgical site infections. Certain strains, such as methicillin-resistant Staphylococcus aureus (MRSA), are resistant to many antibiotics and can be more difficult to treat [5].

Staphylococcus aureus can cause both superficial and subcutaneous infections [6].

• Superficial infections are those that affect the outer layers of the skin. Examples include impetigo, which presents as red sores or blisters, and folliculitis, where hair follicles become inflamed. These infections are usually localized and may involve the epidermis or upper dermis. Furuncle and carbuncle are also superficial infections produced by Staphylococcus aureus.
• Subcutaneous infections extend deeper into the skin, involving the dermis and underlying tissues. These include cellulitis, which cause redness, swelling, and pain, and abscesses, which are pus-filled pockets that may require drainage. Subcutaneous infections can spread to deeper tissues if not treated effectively. Botryomycosis is also an exuberant subcutaneous mycetoma- like lesion, without sinus, produced by Staphylococcus aureus.
• Food is liable for contamination during the various stages of production to consumption cycle i.e. growth, harvesting, processing, transport or even during storage distribution. The contaminant can be physical agent, a chemical agent or a bacterial agent. The consumption of such contaminated food is likely to cause adverse health effect to the consumers and hence refer to as food borne disease or food hazards. Food borne disease are 5 types 1. Food borne intoxication 2. Food borne infection 3. Food borne toxic infection 4. Mycotoxins 5. Food borne diseases due to naturally occurring toxicants.

Food borne intoxication or poisoning are caused by either the

• Ingestion of toxicants that are found in the tissue of certain patient or animals or
• Toxins formed and excreted by bacteria and fungi while they multiple on or in food.

Among Food borne intoxication the most important bacterial food borne intoxication is staphylococcus poisoning.  Staphylococcal food poisoning is one of the most frequently reported food borne disease, which is common throughout the world. The disease is caused by the ingestion of the enterotoxin formed in food, during the growth of certain strains of Staphylococcal aureus, as they multiply in protein rich food. The toxin is referred to as enterotoxin, because it is a toxin produced by bacteria that is specific for intestinal cells and causes gastroenteritis i.e. inflammation of the lining of the intestinal track. Enterotoxin produced by Staphylococcal aureus are designated as A, B, C1, C2, D and E1, the commonly identified toxins involved in the outbreaks are A and D. Small amount of toxin, about 1gm can cause the illness in sensitive persons. Incubation period is generally 2 to 6 hours.

The toxins are heat stable in their crude form. They able to withstand boiling temperatures in the food for several minutes up to 30mins. A number of commercial sterilization process such normal cooling process, spray drying and pasteurization have found not to inactivate the toxin.

After ingestion of a contaminated, the symptoms appear within 1 to 6 hours and in most case, between 2 to 4 hours. The period is dependent on the dose ingested, greater the enterotoxin ingested, short is the time for appearance of the symptoms.

The most important source of Staphylococcal aureus is man. The primary reservoir is the nose, with hair being secondary. This microorganism is found on human skin particularly in areas with cuts, burns, boils etc. which can lead to food contamination. However its presence in cooked or processed food can signal inadequate hygiene practices among food handlers. The foods involved in Staphylococcal aureus food poisoning are typically those that have been handled and then tempt.  Abused prior to consumption.

The most common food implicated in the outbreaks are raw milk, raw meat, custard, cream, bakery foods, poultry and ham, egg foods, fermented meat and dairy products. In India, the dairy product KHOA, the major sweet base, has been involved in several outbreaks.

• Deep seated infections: Abscesses in various sites are deep seated infections caused by S. aureus. Osteomyelitis and exudative pharyngitis are also caused by S. aureus. Left sided endocarditis is also commonly caused by S. aureus. This pathogen is also increasingly being recognized as an important causative agent of community-acquired urinary tract infections, especially in adult males with history of stricture or catheterization.
• Toxin-mediated infections: Toxic shock syndrome is caused by aureus by elaboration of toxins like Toxic shock syndrome toxin. Infections are particularly commonly reported after use of unsterile vaginal tampons. Food poisoning is also commonly caused by S. aureus after ingestion of creamy and sugary food. This occurs due to the Staphylococcal enterotoxin which behaves like super antigen.

Virulence factors of S. aureus can be listed as follows

• Heat stable enterotoxin: This helps in inducing vomiting in cases of Staphylococcal food poisoning.
• Panton-Valentine leucocidin (PVL): This helps kill leukocytes and is dermonecrotic.
• Uronic acid capsule helps in evading phagocytosis.
• TSST-1 or Toxic shock syndrome toxin- 1 helps in inducing features of Toxic shock syndrome like hypotension, vomiting, fever and erythema multiforme.
• Salt tolerance of Staphylococcus aureus helps it survive in tear fluid.
• Staphylococcal cell wall is made up of Pentaglycine linkage, and is hence resistant to the action of lysozyme. Hence it can survive well in tear and saliva, where lysozyme is abundant.
• Staphylococcus aureus readily forms biofilms, which helps evade antibiotics, due to the exopolymeric matrix and upregulated efflux pumps.

Diagnosing Staphylococcus aureus infections involves a combination of clinical evaluation, laboratory testing, and sometimes imaging studies. Here are some of the important diagnostic process. Samples are subjected to culture and Gram stain. Colonies are further identified by staining and biochemical tests.

Microscopic Examination

Gram Staining

• Procedure: A sample from the infection site (e.g., pus, blood, tissue) is stained using the Gram stain technique.
• Observation: S. aureus appears as Gram-positive (purple) cocci, often arranged in clusters resembling bunch of
• Significance: This is a rapid and preliminary method that helps differentiate S. aureus from other bacteria based on its shape and Gram reaction.

Culture Techniques

Isolation on Agar Plates

• Media Used: S. aureus can be isolated on various agar media, including
• Blood Agar: aureus typically produces beta-hemolysis (clear zones around colonies) due to hemolysin production.
• Mannitol Salt Agar (MSA): Selective for staphylococci due to high salt concentration up to 7.5%. S. aureus ferments mannitol, turning the medium yellow.
• Colony Morphology: Colonies are usually round, smooth, and golden-yellow (hence the name aureus, after “Aurum” meaning “gold” in Lati Pigment occurs due to production of carotenoids and xanthines.
• Incubation Conditions: Plates are incubated at 35-37°C for 18-24 hours.
• Significance: Culture is the gold standard for confirming the presence of S. aureus and allows for further testing. Figure 1 below shows the golden yellow, opaque, low convex colonies of aureus from urine samples.

Figure 1: Opaque colonies of S. aureus

Biochemical Tests

Coagulase Test: Slide or Tube Coagulase Test: Detects the production of coagulase enzyme, which causes clotting of plasma by converting fibrinogen to fibrin. S. aureus is coagulase-positive, which differentiates it from other staphylococcal species like S. epidermidis (coagulase-negative).

Slide catalase Test:

• Procedure: A small amount of bacterial colony is picked with corner of flamed cover slip, and mixed with 3% hydrogen peroxide. The production of bubbles indicates catalase activity.
• Significance: Staphylococci are catalase-positive, distinguishing them from streptococci and enterococci, which are catalase-negative.

DNA base Test: Procedure: Detects deoxyribonuclease activity, where S. aureus hydrolyzes DNA, producing a clear zone around the colony on DNase agar.

Thermonuclease Test

• Procedure: Detects thermonuclease (nuc) enzyme production. A positive result further confirms S. aureus identification [7].
• Significance: Biochemical tests help confirm the identification of S. aureus and distinguish it from other staphylococci.
• Phosphatase test: S. aureus is almost always positive for phosphate, which is tested by growing the colonies on phenolphthalein phosphate agar and then exposing colonies to ammonia vapour.

Differentiation from Micrococcus

• This can be attained by some points like fermentative metabolism in Staphylococcus aureus and Oxidative metabolism in Micrococcus, and modified Oxidase reaction (positive in Micrococcus spp.). Also Micrococcus is Furazolidone resistant while Staphylococcus spp. is Furazolidone susceptible as a rule.
• Indole production is negative in Staphylococcus aureus.

Molecular Methods

Polymerase Chain Reaction (PCR)

• Target Genes: PCR can detect genes specific to S. aureus, such as the 16S rRNA gene or nuc gene (encoding thermonuclease) which is highly specific.
• MRSA Detection: PCR can identify the mecA gene, which confers resistance to methicillin by altering penicillin binding protein PBP 2 to PBP-2a. This, indicates methicillin-resistant S. aureus (MRSA).

Multiplex PCR: Procedure: This Can simultaneously detect multiple genes, including toxin genes (e.g., tst for Toxic Shock Syndrome Toxin, eta for exfoliative toxin) [8].

Real-Time PCR

• Procedure: Quantitative method that allows rapid detection and quantification of S. aureus DNA directly from clinical samples.
• Significance: Molecular methods provide rapid and specific identification, especially for detecting MRSA and toxin-producing strains.

Antibiotic Susceptibility Testing

Disk Diffusion (Kirby-Bauer) Method

Procedure: Antibiotic-impregnated disks are placed on an agar plate inoculated with S. aureus. The zone of inhibition around the disks is measured to determine susceptibility.

Minimum Inhibitory Concentration (MIC) Testing

Procedure: Determines the lowest concentration of an antibiotic that inhibits visible bacterial growth. This can be done using broth dilution, E-test strips, or automated systems.

Methicillin Resistance Testing

• Oxacillin/Methicillin Disk: Detects MRSA by observing growth in the presence of methicillin or oxacillin.
• Cefoxitin Disk Test: Cefoxitin is used as a surrogate for detecting mecA-mediated resistance.
• A medium having MHA with 4% NaCl and 6 micrograms per ml Oxacillin helps in screening of MRSA.

Automated Systems

• Examples: VITEK, Phoenix, or Micro Scan systems that provide rapid susceptibility profiles.
• Significance: Determining antibiotic susceptibility is crucial for guiding effective treatment, especially in the context of MRSA infections.

Serological Testing

Antibody Detection

• Procedure: Detects antibodies against S. aureus toxins or other antigens in the patient’s blood. It is less commonly used but can be important in cases like staphylococcal scalded skin syndrome or endocarditis.
• Enzyme-Linked Immunosorbent Assay (ELISA): Procedure: Used for detecting specific antibodies or antigens related to S. aureus in a sample.
• Significance: Serological tests are typically used in research or for specific clinical cases where toxin-mediated diseases are suspected. They are however, not useful for detection of S. aureus

Phage Typing

Procedure: A method used to identify specific strains of S. aureus based on their susceptibility to lysis by certain bacteriophages [9].

Significance: Though largely replaced by molecular typing methods, phage typing was historically significant in epidemiological studies.

MALDI-TOF Mass Spectrometry

Procedure: Matrix-Assisted Laser Desorption/Ionization Time-of-Flight (MALDI-TOF) mass spectrometry is a rapid method for identifying bacteria based on the unique protein profile (fingerprint) of S. aureus.

Significance: This method provides rapid and accurate identification and is increasingly used in clinical microbiology laboratories.

Advanced Molecular Typing Techniques

Pulse-Field Gel Electrophoresis (PFGE)

Procedure: Used for the genotyping of S. aureus strains to track outbreaks and understand epidemiology [10].

Whole Genome Sequencing (WGS)

Procedure: Provides detailed genetic information about S. aureus strains, including resistance genes, virulence factors, and epidemiological data.

Significance: These advanced techniques are particularly valuable in outbreak investigations and research settings.

Immunological Assays

Latex Agglutination Test

Procedure: Detects specific antigens of S. aureus (e.g., protein A or clumping factor) using latex beads coated with antibodies. A positive result shows visible clumping.

Significance: This rapid test aids in the identification of S. aureus directly from culture

Treatment

Antibiotic Therapy: Staphylococcus aureus infections are typically treated with antibiotics, although the choice of antibiotic depends on the strain's susceptibility profile. Methicillin-resistant Staphylococcus aureus (MRSA) is a major concern due to its resistance to many common antibiotics. Commonly used antibiotics include:

Methicillin-sensitive Staphylococcus aureus (MSSA): Typically treated with beta-lactam antibiotics such as penicillin, oxacillin, or nafcillin [7].

MRSA: Requires treatment with antibiotics such as vancomycin, daptomycin, or linezolid. Alternatives may include trimethoprim-sulfamethoxazole or clindamycin, depending on susceptibility and patient tolerance.

Supportive Care: For severe infections, especially those involving abscesses or deep-seated infections, surgical intervention may be necessary to drain pus or remove infected tissue. This helps reduce the bacterial load and aids in recovery [8].

Special note: Small colony variants of Staphylococcus aureus.

Small colony variants (SCVs) of Staphylococcus aureus are a slow-growing subpopulation that forms tiny colonies on agar plates, about one-tenth that of wild type. They often show auxotrophy for compounds like hemin or menadione, which affects their energy production. They often have impaired electron transport, leading to reduced energy production. SCVs are associated with chronic and recurrent infections due to their ability to persist inside host cells and resist certain antibiotics, especially aminoglycosides. Diagnosing SCVs can be challenging due to their small size and slow growth, making them a significant concern in persistent infections like osteomyelitis and device-related infections. Treatment is difficult due to slow growth and formation of biofilms.

• Hygiene: Good personal hygiene is crucial in preventing Staphylococcus aureus infections. This includes:
• Hand Hygiene: consistent and thorough washing of hands with soap and water, or using alcohol-based hand sanitizers.
• Wound Care: Keeping cuts, scrapes, and wounds clean and covered with sterile bandages until they heal [9].
• Nasal application of topical antibiotics like mupirocin targets S. aureus colonization in the nasal passages, a common reservoir for the bacteria, reducing the risk of subsequent infections.

To minimize the spread of bacteria, one should regularly disinfect frequently-touched surfaces and objects. One should avoid sharing personal items such as towels, razors, and clothing to prevent transmission. In healthcare settings, screening for MRSA carriers and implementing decolonization protocols, such as using topical antiseptics like 2% Mupirocin in external nose can significantly reduce the risk of transmission in high-risk environments [9-11].

Staphylococcus aureus is a smart and cunning pathogen which devises a myriad of mechanisms to evade the host’s immune defences, to produce a plethora of infections affecting various organs. Treatment should be preceded by susceptibility testing.

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