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Gram Positive Coccci

Medical Review: No
Date: 2017-11-09

 

Section 01: Staphylococcus

There are two medically important genera of gram-positive cocci: Staphylococcus and Streptococcus. Two of the most important human pathogens, Staphylococcus aureus and Streptococcus pyogenes, are described in this lecture. Staphylococci and streptococci are nonmotile and do not form spores.

Staphylococcus aureus causes abscesses, various pyogenic infections (e.g., endocarditis, septic arthritis, and osteomyelitis), food poisoning, scalded skin syndrome, and toxic shock syndrome. It is one of the most common causes of hospital-acquired pneumonia, septicemia, and surgical-wound infections. It is an important cause of skin infections, such as folliculitis, cellulitis, and impetigo. It is the most common cause of bacterial conjunctivitis.

In Abscess on foot central raised area of whitish pus surrounded by erythema. An abscess is the classic lesion caused by Staphylococcus aureus. In Scalded skin syndrome widespread areas of “rolled up” desquamated skin in infant, aused by an exotoxin produced by Staphylococcus aureus. In Folliculitis multiple small pustules on the skin arises. Impetigo crops the vesicles with honey coloured crust.  

Important properties of Staphylococcus are Staphylococcus aureus gram strain: “grapelike” clusters of gram-positive cocci, Mannitol fermentation: S. aureus ferments mannitol (left) S. epidermidis do not, Haemolysis: Beta haemolysis by Staphylococcus aureus, Catalase Test: All staphylococcus are catalase positive, Coagulase test: Upper tube inoculated with S. aureus, lower tube inoculated with S. epidermidis.

S. aureus has several important cell wall components and antigens: Protein A is the major protein in the cell wall, Teichoic acids are polymers of ribitol phosphate. They mediate adherence of the staphylococci to mucosal cells, Lipoteichoic acids play a role in the induction of septic shock by inducing cytokines such as interleukin-1 (IL-1) and tumor necrosis factor (TNF) from macrophages. Polysaccharide capsule is also an important virulence factor. There are 11 serotypes based on the antigenicity of the capsular polysaccharides. The skin, especially of hospital personnel and patients, is also a common site of S. aureus colonization. Hand contact is an important mode of transmission, and handwashing decreases transmission.

S. aureus causes disease both by producing toxins and by inducing pyogenic inflammation. The typical lesion of S. aureus infection is an abscess. Abscesses undergo central necrosis and usually drain to the outside (e.g., furuncles and boils), but organisms may disseminate via the bloodstream as well. Foreign bodies, such as sutures and intravenous catheters, are important predisposing factors to infection by S. aureus.

 

Section 02: Staphylococcus: Clinical Aspects

The important clinical manifestations caused by S. aureus can be divided into two groups: pyogenic (pus-producing) and toxin-mediated. S. aureus is a major cause of skin, soft tissue, bone, joint, lung, heart, and kidney infections. Pyogenic diseases are the first group described, and toxin-mediated diseases are the second group. Skin infections are very common. These include impetigo, furuncles, carbuncles, paronychia cellulitis. Food poisoning (gastroenteritis) is caused by ingestion of enterotoxin, which is performed in foods and hence has a short incubation period (1–8 hours). In staphylococcal food poisoning, vomiting is typically more prominent than diarrhoea. Toxic shock syndrome is characterized by fever; hypotension; a diffuse, macular, sunburn-like rash that goes on to desquamate; and involvement of three or more of the following organs: liver, kidney, gastrointestinal tract, central nervous system, muscle, or blood. Scalded-skin syndrome is characterized by fever, large bullae, and an erythematous macular rash.

There are two coagulase-negative staphylococci of medical importance: S. epidermidis and S. saprophyticus. S. epidermidis infections are almost always hospital-acquired, whereas S. saprophyticus infections are almost always community-acquired.

Then in this section educator explained the Laboratory Diagnosis. Smears from staphylococcal lesions reveal gram-positive cocci in grapelike clusters. Cultures of S. aureus typically yield golden-yellow colonies that are usually β-hemolytic. S. epidermidis is sensitive, whereas S. saprophyticus is resistant. For Treatment, There are no serologic or skin tests used for the diagnosis of any acute staphylococcal infection. In the United States, 90% or more of S. aureus strains are resistant to penicillin G. Most of these strains produce β-lactamase. Such organisms can be treated with β- lactamase–resistant penicillin’s (e.g., nafcillin or cloxacillin), some cephalosporins, or vancomycin. Treatment with a combination of a β-lactamase–sensitive penicillin (e.g., amoxicillin) and a β-lactamase inhibitor (e.g., clavulanic acid) is also useful.

For Prevention there is no vaccine against staphylococci. Cleanliness, frequent handwashing, and aseptic management of lesions help to control spread of S. aureus. Persistent colonization of the nose by S. aureus can be reduced by intranasal mupirocin or by oral antibiotics, such as ciprofloxacin or trimethoprim-sulfamethoxazole, but is difficult to eliminate completely. Shedders may have to be removed from high-risk areas. Cefazolin is often used perioperatively to prevent staphylococcal surgical-wound infections.

 

Section 03: Streptococcus

Streptococci cause a wide variety of infections. S. pyogenes (group A streptococcus) is the leading bacterial cause of pharyngitis and cellulitis. It is an important cause of impetigo, necrotizing fasciitis, and streptococcal toxic shock syndrome. It is also the inciting factor of two important immunologic diseases, namely, rheumatic fever and acute glomerulonephritis. Streptococcus agalactia (group B streptococcus) is the leading cause of neonatal sepsis and meningitis. Enterococcus faecalis is an important cause of hospital-acquired urinary tract infections and endocarditis. Viridans group streptococci are the most common cause of endocarditis. Streptococcus bovis also causes endocarditis.

Classification of Streptococci are: Beta haemolytic streptococci involves Group A-U (Lancefield group) and Precipitin test, Immunofluorescence, and Non beta haemolytic streptococci involves Alpha haemolytic and Gamma haemolytic. Important properties of Streptococci are: Long chain of streptococci, Catalase test: Streptococci are catalse negative, Haemolysis: Alpha haemolysis, beta hemolysis and Gamma hemolysis by Enterococcus faecalis. In explanting Transmission educator tells that the most streptococci are part of the normal flora of the human throat, skin, and intestines but produce disease when they gain access to tissues or blood. Viridans streptococci and S. pneumoniae are found chiefly in the oropharynx; S. pyogenes is found on the skin and in the oropharynx in small numbers; S. agalactiae occurs in the vagina and colon; and both the enterococci and anaerobic streptococci are located in the colon. Group A streptococci (S. pyogenes) cause disease by three mechanisms: (1) pyogenic inflammation, which is induced locally at the site of the organisms in tissue; (2) exotoxin production, which can cause widespread systemic symptoms in areas of the body where there are no organisms; and (3) immunologic, which occurs when antibody against a component of the organism cross-reacts with normal tissue or forms immune complexes that damage normal tissue

 

Section 04: Streptococcus: Clinical Aspects

In the start of section four educator describes the Clinical Findings. S. pyogenes causes three types of diseases: (1) pyogenic diseases such as pharyngitis and cellulitis, (2) toxigenic diseases such as scarlet fever and toxic shock syndrome, and (3) immunologic diseases such as rheumatic fever and acute glomerulonephritis (AGN).

Then the educator explains the Laboratory Diagnosis. Gram-stained smears are useless in streptococcal pharyngitis because viridians streptococci are members of the normal flora and cannot be visually distinguished from the pathogenic S. pyogenes. However, stained smears from skin lesions or wounds that reveal streptococci are diagnostic. Cultures of swabs from the pharynx or lesion on blood agar plates show small, translucent β-haemolytic colonies in 18 to 48 hours. If inhibited by bacitracin disk, they are likely to be group A streptococci. Then educator explains in detail the preventions, and for that no vaccines except for S pneumonia.

 

Section 05: Streptococcus Pneumonia

Streptococcus pneumoniae causes pneumonia, bacteremia, meningitis, and infections of the upper respiratory tract such as otitis media, mastoiditis, and sinusitis. Pneumococci are the most common cause of community-acquired pneumonia, meningitis, sepsis in splenectomised individuals, otitis media, and sinusitis. Important Properties involves Pneumococci are gram-positive lancet-shaped cocci arranged in pairs (diplococci) or short chains. Optochin Test: Lower half of plate shows alpha hemolysis by S pneumonia except in the region around optochin disc, Pneumococci possess polysaccharide capsules of more than 85 antigenically distinct types. With type-specific antiserum, capsules swell (quellung reaction), and this can be used to identify the type. Another important surface component of S. pneumoniae is a teichoic acid in the cell wall called C-substance (also known as C-polysaccharide).

Humans are the natural hosts for pneumococci; there is no animal reservoir. Because a proportion (5%–50%) of the healthy population harbours virulent organisms in the oropharynx, pneumococcal infections are not considered to be communicable. Resistance is high in healthy young people, and disease results most often when predisposing factors (see below) are present.

The most important virulence factor is the capsular polysaccharide, and anti capsular antibody is protective. Lipoteichoic acid, which activates complement and induces inflammatory cytokine production, contributes to the inflammatory response and to the septic shock syndrome that occurs in some immunocompromised patients. Pneumolysin, the hemolysin that causes α-hemolysis, may also contribute to pathogenesis. 

Khurshid, Aqsa
  • Academics: MS
  • Specialization: Industrial Biotechnology
  • Current: Senior Technologist
  • Hospital: AFIP
  • Location: Islamabad, Pakistan
  •   
  • Course: Microbiology
  • Clinical Years: 2
  • Teaching Years: 1

Ms. Aqsa Khurshid did her Masters of science in 2014 from National University of Science and Technology. And after that, she is serving at Armed Forces Institute of Pathology (AFIP) as senior technologist.

She also has publications in Global Journal of Medical Research.


4 lectures