Staphylococci PDF

exchange.scholarrx.com /brick/staphylococci Staphylococci 17-22 minutes Staphylococcus is a genus of gram-positive, non–spore-forming bacteria that includes some of the most common infections encountered by physicians. Staphylococci (or staph for short) include more than 40 species, 5 of which are c...

exchange.scholarrx.com /brick/staphylococci Staphylococci 17-22 minutes Staphylococcus is a genus of gram-positive, non–spore-forming bacteria that includes some of the most common infections encountered by physicians. Staphylococci (or staph for short) include more than 40 species, 5 of which are considered potential human pathogens: S aureus, S epidermidis, S saprophyticus, S haemolyticus, and S hominis. Our focus will be on the first three of these. Despite the infectious propensity of staphylococci, it would be unfair to call them strictly pathogenic (or bad) bacteria. In fact, most are part of our normal skin flora and do not usually cause harm when they exist in this state. Though they are in the same genus and share some characteristics, each species has its own distinguishable living conditions, virulence factors, and associated infections. Are all 40+ species of staphylococci capable of causing infection in humans? No, only five are capable of causing infection in humans. Staphylococci have a unique appearance on Gram stain and stain purple. They are spherical in shape and tend to cluster together like grapes (Figure 1). Figure 1 Credit: Image courtesy of Y Tambe. What do staphylococci look like on Gram stain? They appear as purple, grape-like clusters. All staphylococci are facultative anaerobic organisms, meaning they are capable of both aerobic and anaerobic growth. They are also all catalase positive (meaning they produce the enzyme catalase), so they are able to convert hydrogen peroxide (H2O2) to water and oxygen. This makes the catalase test useful to distinguish Staphylococcus from Enterococcus and Streptococcus (Figure 2). Figure 2 Credit: ©ScholarRx Because all the staph species look similar on Gram stain, laboratory tests and selective media are needed to identify each of them. They are detailed next and summarized in Table 1. Table 1 S aureus S epidermidis S saprophyticus Catalase Positive Positive Positive Coagulase Positive Negative Negative Blood agar hemolysis β-Hemolytic γ-Hemolytic (nonhemolytic) γ-Hemolytic (nonhemolytic) Mannitol fermentation Ferments Does not ferment Does not ferment Deoxyribonuclease (DNase) Positive Negative Negative Bile salt sensitivity Resistant Resistant Resistant Novobiocin sensitivity Sensitive Sensitive Resistant Coagulase Test. One of the most important features used in the classification of staphylococci is their ability to produce coagulase, which is an enzyme that causes blood clot formation. In a positive coagulase test, the enzyme creates an insoluble fibrin capsule around the microorganism. This is a dead giveaway when classifying S aureus from other forms of staphylococci. Hemolysis. Of the three species mentioned, only S aureus is β-hemolytic, making this test another useful way to separate it from other staphylococci (Figure 3). Culturing on blood agar reveals complete lysis of red blood cells, leaving a pale or clear area surrounding the S aureus colonies. Figure 3 Credit: Image courtesy of Y Tambe. Mannitol Salt Agar. Mannitol salt agar (MSA) is selective for gram-positive bacteria. For Staphylococcus, it usually contains 7.5% sodium chloride in the media. S aureus can ferment mannitol and will turn yellow on MSA (due to staphyloxanthin pigment, a carotenoid similar to vitamin A found in carrots and other yellow vegetables), whereas other staphylococci produce small pink or red colonies without color change to the media because they do not ferment mannitol (Figure 4). Figure 4 Credit: Courtesy of Navaho. To remember how to identify S aureus, think: S aureus has a yellow pigment. Aureus is Latin for gold. How you remember what color helps identify S aureus? Deoxyribonuclease Test. DNA is hydrolyzed into oligonucleotides by the action of deoxyribonuclease (DNase). In the test, acids are used. DNA is insoluble in acid, and nucleotides are soluble. S aureus produces DNase, while S epidermidis and S saprophyticus (and most staphylococci) do not have this enzyme. Bile Salt Resistant. All staphylococci grow in the presence of bile salts. Novobiocin Test. Novobiocin is an antibiotic that best differentiates S saprophyticus from similar bacteria. S aureus and S epidermidis are both sensitive to novobiocin, but S saprophyticus is distinctly resistant. Which species of Staphylococcus is catalase positive, coagulase negative, and novobiocin resistant? S saprophyticus is a catalase-positive, coagulase-negative, and novobiocin-resistant bacterium. Staphylococci are common commensal colonizers of body surfaces and are therefore common causes of bacterial infections of the skin. As commensals, they don’t normally cause harm when they are in their ecologic niche but do cause problems when they grow elsewhere. Breaks in our protective skin layer, such as from injury, create the opportunity for bacteria to cause infections, because they end up deeper in our body than they are supposed to be. S aureus can be part of the normal flora of human skin but are more commonly found as normal flora in the anterior nares (nostrils). Transient colonization of moist skin folds and the nasopharyngeal cavity is also common. S aureus also survive for long periods on dry surfaces. As the name implies, S epidermidis is a commensal of the skin but can cause severe endogenous infections in immunosuppressed patients and those with central venous catheters and Foley catheters. S saprophyticus is part of the normal vaginal flora. It is predominantly implicated in genitourinary (GU) tract infections in sexually active young women. Where are S aureus, S epidermis, and S saprophyticus normally found in the human body? S aureus, S epidermis, and S saprophyticus are normally found in the nose, on the skin, and in the vagina, respectively. Now that we know where staph come from, we can see how infections by these bacteria vary depending on the species and their origin. We mentioned that this can be the result of an opportunistic infection that occurs when normal flora are able to gain entry to a normally sterile environment. We classify these as endogenous infections. Infections can also be due to exogenous factors such as direct bacterial spread from person to person, encounter of fomites on surfaces, or even contaminated food. These exogenous infections are often further classified by whether they were acquired in the community or as health care–associated infections (HAIs). The most dangerous bacteria we must be concerned about is S aureus. Table 2 shows where each species typically colonizes and also classifies the most important infections caused by our three main bacteria to be discussed in more detail below. As you can see, some of the staphylococci are capable of inducing the same infectious course, and similarly, S epidermidis is capable of inducing the same infectious course in multiple settings. Table 2 Bacteria Colonization site S aureus S epidermidis Skin, nares, in folds throughout the body Skin S saprophyticus Urogenital Cellulitis Endogenous infections Osteomyelitis and septic arthritis Impetigo Abscesses and similar lesions Toxic shock syndrome Secondary pneumonia in susceptible populations Community acquired Cellulitis Osteomyelitis Endocarditis UTIs UTIs Impetigo Acute infective endocarditis Food poisoning Secondary pneumonia (post-viral infection) Ventilator-associated bacterial Nosocomial, iatrogenic pneumonia Surgical site infections Deep tissue infections Osteomyelitis (in joint replacement) Subacute endocarditis Soft tissue infections (eg, cellulitis) are caused commonly by S aureus and, much less commonly, S epidermidis. They can have debilitating consequences if not controlled. Abscesses are confined pockets of pus that collect in tissues, organs, or other spaces in the body. They are most commonly caused by S aureus and may appear with fever and localized pain. Cellulitis is an acute infection of the dermis and subcutaneous tissues. It usually starts from a break in the skin from trauma or another infection. Folliculitis is an infection of the hair follicle. It appears as a tender pustule. Furuncles are small abscesses exuding purulent material from a single opening that involve both the skin and the subcutaneous tissues in areas with hair follicles. Impetigo is a common superficial skin infection consisting of highly contagious honey crusted lesions and is usually caused by S aureus. Bullous impetigo is a similar but less contagious condition in which fluidfilled bullae (or blisters) form at a local site of infection (Figure 5). This too is caused commonly by S aureus; however, the bacteria’s production of an exfoliative toxin is what leads to the fluid-filled lesion type. Osteomyelitis is an infection of the bone. Many organisms can cause osteomyelitis, but S aureus is the most common. Figure 5 Credit: Courtesy of Dr. Richard Usatine Staphylococcus can affect many other organs once the bacteria are introduced to areas that are normally sterile. The routes of these invasive infections are diverse and are not limited to those described below. Contamination of Medical Devices. Medical devices such as prosthetic valves, pacemakers, central lines, intravascular (indwelling) catheters, and urinary catheters are commonly contaminated with S epidermidis. Although S aureus can also colonize foreign bodies, S epidermidis is more classically associated with colonization of foreign bodies and iatrogenic infections (infections introduced inadvertently by a clinician during medical procedures): Subacute endocarditis in patients with a history of heart defects or valve replacements is usually caused by S epidermidis; it may have few to no symptoms. Acute endocarditis is a rapid-onset condition involving large vegetations on both normal and diseased heart valves. It most commonly affects the tricuspid valve and is most often seen with unsterile intravenous (IV) drug use. S aureus is the most common cause for acute cases. Thrombophlebitis is inflammation of a vein related to a thrombus. Septic or suppurative thrombophlebitis is categorized by fever, pain, and (occasionally) erythema at the insertion site of an IV catheter. This usually affects hospitalized patients and can be caused by S epidermidis and S aureus. Hematogenous spread is related to the ability of staphylococci to travel in the bloodstream and infect multiple organs. This is also called bacteremia and can lead to septic shock. Urinary tract infections (UTIs) related to catheter use are commonly caused by S epidermidis because of its tendency to create biofilms on medical devices. UTIs can be prevented by routine removal, sterilization, and replacement of urinary catheters. What species of staphylococci normally infects abnormal or replaced heart valves? Which species attacks normal heart valves? S epidermidis normally attacks diseased valves. S aureus more commonly infects normal heart valves. Staphylococcal Pneumonia. S aureus is most commonly acquired in the hospital, especially by patients who have been on a ventilator. Secondary causes of pneumonia happen as a result of another illness such as influenza or cystic fibrosis. This is the most common way S aureus will cause community-acquired pneumonia and is considered during influenza epidemics or if the patient has a recent history of viral pneumonia. S aureus is estimated to cause 1%-10% of community-acquired pneumonias and 20%-50% of health care–associated pneumonias. Staphylococcal pneumonia is usually a bronchopneumonia that is multifocal and often bilateral. Strict lobar involvement is unusual. In an acute phase, it may cause cavitary lesions or pleural effusion and is associated with lung abscesses and empyema. If the strain of S aureus produces the pore-forming toxin Panton-Valentine leukocidin (PVL), it can cause necrotizing pneumonia. In necrotizing pneumonia, previously healthy children or young adults can initially present with influenza-like symptoms that rapidly worsen to respiratory failure and septic shock. If therapy is not started early, mortality rates are very high. Clearly, S aureus causes the most dangerous infections. Remember, it is important to think about S aureus as the cause of secondary pneumonia in post-influenza patients and hospital-acquired bacterial pneumonia after long periods of ventilator support. In addition to pneumonia, S aureus should also be considered when working up cases involving cellulitis, osteomyelitis, septic arthritis, impetigo, GI intoxications, carbuncles, folliculitis, and acute infective endocarditis. Let’s understand why this species of bacteria holds such great potential to disrupt our homeostasis. So how does S aureus wreak so much havoc when it leaves its niche? Disease processes with these bacteria are numerous, and the portal of entry is variable (eg, skin, respiratory tract, GU tract). S aureus expresses many potential virulence factors: Surface proteins: promote colonization of host tissues Leukocidin, kinases, hyaluronidase: invasins that promote bacterial spread in tissues; PVL is an important cause of necrotizing pneumonia Capsule, protein A: surface factors that inhibit phagocytic engulfment Carotenoids, catalase: enhance staphylococcal survival in phagocytes Protein A, coagulase: immunological disguises Hemolysins, leukotoxin, leukocidin: membrane-damaging toxins that lyse eukaryotic cell membranes Exotoxins: damage host tissues or otherwise provoke symptoms of disease Inherent and acquired resistance to antimicrobial agents The most important virulence factors that contribute most to its pathogenicity and are unique to S aureus are detailed below. These are the main contributors to a fulminant inflammatory response. Protein A. One of the major virulence factors for S aureus is protein A, which binds the Fc region of immunoglobulin G (IgG), thereby inhibiting complement fixation and phagocytosis. What virulence factor is unique to S aureus and helps resist phagocytosis? Protein A is the unique virulence factor that helps S aureus resist phagocytosis. Exotoxins. S aureus exotoxin-mediated diseases include toxic shock syndrome (TSS), food poisoning, bullous impetigo, and scalded skin syndrome. S aureus infections can be treated with antibiotics, but exotoxin-mediated effects persist. The toxins produced include toxic shock syndrome toxin (TSST), enterotoxin (ET), and exfoliatin and are discussed next. TSST-1. TSST-1 causes TSS and is historically associated with tampon use and foreign bodies (such as nasal packing for a nose bleed or postsurgical status). Signs and symptoms of TSS include: Fever Hypotension: dizziness, multi-organ failure Nausea and vomiting Rash: diffuse erythema that starts on the trunk and spreads to the extremities, erythema of palms and soles, conjunctival hyperemia, and strawberry tongue; 1-2 weeks later, the rash will desquamate, especially on the palms and soles Enterotoxin. Enterotoxin is heat resistant (stable at 100°C for 1 hour) and causes rapid-onset food poisoning. Symptoms arise within 1-6 hours of ingesting contaminated food and predominantly include nausea and vomiting; watery diarrhea may also occur. Exfoliatin. Exfoliatin is a proteolytic exotoxin that cleaves the desmosomes, causing a blister just below the stratum corneum of the epidermal skin. This exotoxin plays a role in the pathogenesis of bullous impetigo and staphylococcal scalded-skin syndrome (Figure 6). Figure 6 Credit: Courtesy of Duijsters CE, Halbertsma FJ, Kornelisse RF, et al. Superantigens. Superantigens are extremely powerful exotoxins that initiate potent activation of the immune response by facilitating the binding of major histocompatibility complex (MHC) class II molecules on antigenpresenting cells with the T-cell receptor of CD4 T cells. This interaction can occur without the presence of a peptide in the T-cell receptor groove and results in a significant release of cytokines. TSST-1 and enterotoxin are both superantigens. Which S aureus toxins are superantigens? TSST and enterotoxin are superantigens. S epidermidis is coagulase negative, urease positive, and nonhemolytic with white colonies. It is novobiocin sensitive (unlike S saprophyticus, which is novobiocin resistant). S epidermidis can generate a sticky, protective biofilm, facilitating colonization and infection of prosthetic devices like prosthetic joints, heart valves, and indwelling urinary catheters. Prosthetic valve endocarditis during the initial year after surgery is almost exclusively caused by S epidermidis. What feature of S epidermidis facilitates colonization and infection of medical devices such as implants? Biofilms facilitate colonization on medical devices. S saprophyticus is coagulase negative, urease positive, and novobiocin resistant. It causes UTIs and cystitis. Infections spread via bodily fluids and often occur in sexually active young women. Practices and characteristics that increase risk of infection include unprotected sex, a short urethra, and open cuts or sores. S saprophyticus is part of the normal flora of the rectum and vagina in up to 10% of females. Patients typically present with cystitis (symptoms include dysuria, polyuria, and abdominal pain or discomfort). Laboratory diagnostics include urinalysis and culture (collect sample midstream and culture using blood agar). In short, all of them can be, but the most common one you’ll hear about is S aureus. Most S aureus strains are resistant to conventional penicillin because of their ability to produce penicillinase enzyme. Bacterial penicillinase can cleave penicillin’s B-lactam ring and thus making the drug ineffective. Methicillin, the prototypical anti-staphylococcal penicillin, is no longer manufactured as a treatment drug but is still used in characterizing S aureus isolates resistant to it. Methicillin resistance is conferred by altered penicillin-binding proteins (encoded by the MecA gene) in the plasmid. Methicillin-resistant S aureus (MRSA) accounts for about 40% of all HAI S aureus infections in the United States. Vancomycin has been commonly used to treat antibiotic-resistant bacteria like MRSA, but vancomycinintermediate S aureus (VISA) and vancomycin-resistant S aureus (VRSA) strains are emerging due to its repeated use. VRSA strains acquired the vanA gene from enterococci, a somewhat incredible feat, as it represents genetic transmission between bacterial genera. The vanA gene changes the amino acids on the muramic acid side chain, causing steric hindrance, decreasing the ability of vancomycin to adequately bind. Thankfully, at the time of this writing, only 14 VRSA infections have been reported in the United States, dating back to the first case reported in 2002. A minimum inhibitory concentration (MIC) is used to determine if bacteria are resistant to antimicrobial agents that might be used for treatment of infections. Staph are classified as VISA if the MIC for vancomycin is 4-8 µg/mL and classified as VRSA if the vancomycin MIC is ≥16 µg/mL.