staphylococcus species

what is Staphylococcus?

gram-positive spherical bacteria that is arranged in grape like clusters

what is the oxygen requirement of staphylococci?

facultative anaerobes

how can staphylococci be differentiated from streptococci?

the catalase test, which is done by taking a small amount of the bacterial colony and placing it on a clean glass slide, then adding a drop of 3% hydrogen peroxide (H₂O₂). if bubbles form, the test is catalase positive, indicating Staphylococcus. If no bubbles form, the test is catalase negative, indicating Streptococcus. The bubbles are oxygen produced when catalase breaks down hydrogen peroxide into water and oxygen.

how can pathogenic bacteria be identified?

• microscopic smears (bacterial morphology and arrangement)

• cultural characteristics (bacterial growth features on/in solid or liquid media)

• biochemical characteristics (metabolism activity such as endo or exoenzymes)

• serological reactions (antigen-antibody reaction)

• molecular sequence (polymerase chain reaction PCR)

how can staphylococci further be divided?

based on if they produce staphylocoagulase, it is divided into 2:

• coagulase positive

• coagulase negative

what is staphylococcus aureus?

coagulase-positive and has the greatest pathogenic potential and is the most medically important member of the genus. it is found in the anterior nare (main habitat)

what is staphylococcus epidermidis?

coagulase-negative staphylococci, which is far less pathogenic than S.aureus. main habitat is skin

what are characteristics of Staphylococcus aureus?

like other staphylococci it is gram-positive cocci arranged in grape like clusters and is coagulase positive

what does Staphylococcus aureus grow on?

• nutrient ager, forming golden yellow pigmented colonies

• blood ager, producing colonies surrounded with a zone of hemolysis (complete hemolysis)

what are important virulence factors Staphylococcus aureus?

• staphylocoagulase

• lactamase

what is staphylocoagulase?

coagulase is an extracellular enzyme that has the ability to convert plasma fibrinogen to fibrin. by this mechanism, a fibrin barrier is formed which leads to protection of bacteria from phagocytes and immune defenses, as well as localization of infection

what is lactamase?

an enzyme which breaks down beta lactam antibiotics, which plays a role in antibiotic resistance

what are localized pyogenic infections caused by staphylococcus aureus?

• abscesses, folliculitis, furuncles or carbuncles

• sialadenitis, which is the painful infection of the salivary gland

• postoperative surgical or traumatic wound infections

what are invasive infections caused by staphylococcus aureus?

due to invasion of bloodstream (bacteremia), then spreading to numerous body sites, for example, pneumonia, osteomyelitis, arthritis and endocarditis

how is the laboratory diagnosis of S.aureus done?

first, a direct gram-stained smear is done, and it is typically seen as a gram-positive cocci arranged in clusters (purple) and is seen near or among inflammatory cells. however, microscopy cannot differentiate S. aureus from other species

what must be done to identify S. aureus?

the specimen is cultured on nutrient agar and blood ager. on nutrient ager, the colonies will appear golden yellow and the blood ager will be surrounded with a zone of hemolysis. a catalase test is done, and if bubbling and positive, that confirms it is Staphylococcus. to further identify it, a coagulase test is done. if positive, that means it is S. aureus

what is of primary important for the treatment of staphylococcal infections?

beta-lactam antibiotics such as penicillin’s, cephalosporin’s, carbapenems and monobactams, however, the face serious antibiotic resistance patterns of S. aureus

what is penicillin resistant S. aureus?

approximately 95% of S. aureus strains are now resistant to penicillin due to the production of beta-lactamase (penicillinase) enzyme

strains resistant to penicillin remain susceptible to what?

cephalosporin’s, carbapenems and monobactams

what are methicillin resistant S. aureus (MRSA)?

it is a more serious type of resistance and occurs due to expression of new penicillin binding proteins (PBPs), which are binding sites for the antibiotic on the organism’s cell wall. infection caused by MRSA strains cannot be treated with any type of beta-lactam antibiotics

what is the drug of choice for MRSA?

vancomycin

what are vancomycin-resistant S. aureus?

during the last few years, some strains of MRSA have displayed intermediate (VISA) or full resistance (VRSA) to vancomycin, which is very determinantal as vancomycin is considered the most reliable and effective treatment against MRSA

what antibiotics are used for treatment of S.aureus infections not responding to vancomycin?

• linezolid

• streptogramins