Coagulase negative staphylococci & Streptococcus · and Streptococcus mutans) are not bile-soluble...

Microbiology

Coagulase negative staphylococci & Streptococcus

Dr. Abdullah Ahmed Hama

PhD. Microbiology/Parasitology

[email protected]

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Dr. Abdullah A. Hama Microbiology Text: Jawetz, & Adelberg's 2010

Sulaimani University College of Pharmacy

mailto:[email protected]

mk:@MSITStore:D:/cryptosporidium all/book parasitology/Jawetz, Melnick, & Adelberg's Medical Microbiology, 23rd Edition by Vishal.chm::/www.accessmedicine.com/resourcetoc.aspx@resourceid=6




Other than S. aureus, there are more than 30 species of staphylococci.

• In medical practice, the 15 species that have been isolated from human infections are typically lumped together by a negative characteristic—failure to produce coagulase.

• There are two coagulase-negative staphylococci of medical importance:

S. epidermidis

S. saprophyticus.

• Unlike S. aureus, these two coagulase-negative staphylococci do not produce exotoxins. Thus, they do not cause food poisoning or toxic shock syndrome.

• They have been shown to have surface adhesins and the ability to produce extracellular polysaccharide biofilms.

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Staphylococcus epidermidis • S. epidermidis and many other species of CONS are normal

commensals of the skin, anterior nares, and ear canals of humans.

• S. epidermidis infections are almost always hospital-acquired, whereas S. saprophyticus infections are almost always community-acquired.

• S. epidermidis is part of the normal human flora on the skin and mucous membranes but can enter the bloodstream (bacteremia) and cause metastatic infections, especially at the site of implants. It commonly infects intravenous catheters and prosthetic implants, e.g., prosthetic heart valves (endocarditis), vascular grafts, and prosthetic joints (arthritis or osteomyelitis).

• S. epidermidis is also a major cause of sepsis in neonates and of peritonitis in patients with renal failure who are undergoing peritoneal dialysis through an indwelling catheter.

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• The biology of S saphrophyticus infection is entirely different. Its usual habitat is the gastrointestinal tract, and from that location the organism gains access to the urinary tract.

• Among sexually active women, S saphrophyticus is second only to Escherichia coli as a cause of acute urinary tract infection.

• This process may be aided by surface adhesins to uroepithelial cells and the production of a urease.

• Simple test (novobiocin resistance) is often used to separate S. saprophyticus from other urinary isolates.

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Staphylococcus saphrophyticus


S. epidermidis (S)

S. saprophyticus (R)





Streptococci: • Streptococci are spherical gram-positive cocci arranged in chains or pairs.

• All streptococci are catalase-negative, whereas staphylococci are catalase-positive.

• Streptococci are smaller and more ovoid in shape than staphylococci

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Three types of hemolysis are present and varies by species: 1. Beta = clear (complete hemolysis). 2. Alpha = partial hemolysis (green). 3. Gamma = non-hemolytic.





Streptococcus

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Cultural and Biochemical Characteristics

• Grow best under aerobic or anaerobic conditions (facultative).

• Blood agar is preferred because it satisfies the growth requirements and also serves as an indicator for patterns of hemolysis.

• The colonies are small (2 mm in diameter), and they may be surrounded by a zone where the erythrocytes suspended in agar have been hemolyzed.

• When the zone is clear, this state is called β-hemolysis.

• When the zone is hazy with a green discoloration of the agar, it is called α-hemolysis

• Catalase negative.





Classification:

The classification of streptococcus is based on hemolysis and biochemical tests:

• Lancefield classification by serogroups (eg, A, B, C)

• Presence of Lancefield antigens defines the group of streptococci.

• Rebecca Lancefield, who demonstrated carbohydrate antigens in cell-wall extracts of the β-hemolytic streptococci, put this taxonomy on a sounder basis.

• One of the most important characteristics for identification of streptococci is the type of hemolysis:

1. Alpha-hemolytic streptococci form a green zone around their colonies as a result of incomplete lysis of RBCs in the agar.

2. Beta-hemolytic streptococci form a clear zone around their colonies because complete lysis of the RBCs. Beta-hemolysis enzymes (hemolysins) called streptolysin O and streptolysin S .

3. Some streptococci are non-hemolytic (gamma-hemolysis).

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Streptococcus classification The streptococci are considered here as follows:

• pyogenic streptococci

• Enterococci Beta-Hemolytic Streptococci

• pneumococcus

• viridans Non-Beta-Hemolytic Streptococci

•

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Beta-Hemolytic Streptococci • These are arranged into groups A–U (known as Lancefield groups)

on the basis of antigenic differences in C carbohydrate.

• Of the many Lancefield groups, groups A (S pyogenes) and B (S agalactiae) are the most common causes of serious disease.

• The group D carbohydrate is found in the genus Enterococcus, which used to be classified among the streptococci.

• There are two important antigens of beta-hemolytic streptococci:

1. C carbohydrate determines the group of beta-hemolytic streptococci. It is located in the cell wall.

2. M protein is the most important virulence factor and determines the type of group A beta-hemolytic streptococci. It protrudes from the outer surface of the cell and interferes with ingestion by phagocytes.

• There are approximately 80 serotypes based on the M protein, which explains why multiple infections with S. pyogenes can occur.

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Group A streptococci (S. pyogenes) are one of the most important human pathogens.

• They are the most frequent bacterial cause of pharyngitis and a very common cause of skin infections.

• They adhere to pharyngeal epithelium via pili covered with lipoteichoic acid and M protein.

• Many strains have a hyaluronic acid capsule that is antiphagocytic.

• The growth of S. pyogenes is inhibited by the antibiotic bacitracin, an important diagnostic criterion

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Group B streptococci (S. agalactiae) : Colonize the genital tract of some women and can cause neonatal meningitis and sepsis. • They are usually bacitracin-resistant. • They hydrolyze (breakdown) hippurate, an important diagnostic

criterion.





Group D streptococci (enterococci)

• (e.g., Enterococcus faecalis and Enterococcus faecium).

• Enterococci are members of the normal flora of the colon and are

noted for their ability to cause urinary, biliary, and cardiovascular

infections.

• They are very hardy organisms; they can grow in hypertonic (6.5%)

saline or in bile and are not killed by penicillin G.

• Vancomycin-resistant enterococci (VRE) have emerged and become

an important and much feared cause of life-threatening nosocomial

infections. More strains of E. faecium are vancomycin-resistant than

are strains of E. faecalis. 11






Non-enterococcal group D streptococci,

• such as S. bovis, can cause similar infections but are much less

hardy organisms; for example, they are inhibited by 6.5% NaCl and

killed by penicillin G.

• Note that the hemolytic reaction of group D streptococci is

variable: most are alpha-hemolytic and beta-hemolytic and

others are non-hemolytic.

• Groups C, E, F, G, H, and K–U streptococci infrequently cause

human disease.

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Non-Beta-Hemolytic Streptococci • Some produce no hemolysis; others produce alpha-hemolysis.

• The principal alpha-hemolytic organisms are S. pneumoniae and the

viridans group of streptococci.

• The viridans streptococci (e.g., Streptococcus mitis, Streptococcus sanguis,

and Streptococcus mutans) are not bile-soluble and not inhibited by

optochin—in contrast to S. pneumoniae, which is bile-soluble and

inhibited by optochin.

• Viridans streptococci are part of the normal flora of the human pharynx

and intermittently reach the bloodstream to cause infective endocarditis.

S. mutans synthesizes polysaccharides (dextrans) that are found in dental

plaque and lead to dental caries. 13






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Streptococcus pyogenes (Group A Streptococci) 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. Biologically Active Products of GAS: The M protein of S. pyogenes is its most important antiphagocytic factor, Capsule, composed of hyaluronic acid, is also antiphagocytic. Antibodies are not formed against the capsule because hyaluronic acid is a normal component of the body and humans are tolerant to it.


Disease





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Group A streptococci produce three important inflammation-related

enzymes:

1. Hyaluronidase degrades hyaluronic acid, which is the ground

substance of subcutaneous tissue. Hyaluronidase is known as

spreading factor because it facilitates the rapid spread of S.

pyogenes in skin infections (cellulitis).

2. Streptokinase (fibrinolysin) activates plasminogen to form plasmin,

which dissolves fibrin in clots, thrombi, and emboli.

3. DNase (streptodornase) degrades DNA in exudates or necrotic

tissue. Streptokinase-streptodornase mixtures applied as a skin

test give a positive reaction in most adults, indicating normal cell-

mediated immunity





Toxin of streptococcus Groupe A In addition, group A streptococci produce five important toxins and

hemolysins: 1. Erythrogenic toxin causes the rash of scarlet fever. Its mechanism of action

is similar to that of the toxic shock syndrome toxin (TSST) of S. aureus; i.e., it acts as a superantigen.

1. Streptolysin O is a hemolysin that is inactivated by oxidation (oxygen-labile). It causes beta-hemolysis only when colonies grow under the surface of a blood agar plate. It is antigenic, and antibody to it (ASO) develops after group A streptococcal infections. The titer of ASO antibody can be important in the diagnosis of rheumatic fever.

2. Streptolysin S is a hemolysin that is not inactivated by oxygen (oxygen-stable). It is not antigenic but is responsible for beta-hemolysis when colonies grow on the surface of a blood agar plate.

3. Pyogenic exotoxin A is the toxin responsible for most cases of streptococcal toxic shock syndrome. It has the same mode of action as does staphylococcal TSST; i.e., it is a superantigen that causes the release of large amounts of cytokines from helper T cells and macrophages.

4. Exotoxin B is a protease that rapidly destroys tissue and is produced in large amounts by the strains of S. pyogenes, the so-called "flesh-eating" streptococci that cause necrotizing fasciitis. 16






Groupe A Streptococcus diseases: 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. Clinical findings: • Streptococcal Pharyngitis • Impetigo • cellulitis • Puerperal Infection • Scarlet Fever • Streptococcal Toxic Shock Syndrome(STSS) • Poststreptococcal Sequelae A)Acute Rheumatic Fever (ARF): B)Acute Glomerulonephritis

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Diagnosis:

• Blood agar plates demonstration of β-hemolysis.

• Bacitracin susceptibility predicts group A.

• Several serologic tests aid in the diagnosis of poststreptococcal sequelae.

Treatment:

• All group A streptococci are susceptible to penicillin G, but neither rheumatic fever nor AGN patients benefit from penicillin treatment after onset.

• In penicillin-allergic patients, erythromycin or one of its long-acting derivatives, e.g., azithromycin, can be used.





Coagulase negative staphylococci & Streptococcus · and Streptococcus mutans) are not bile-soluble...

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