Gram positive bacteria, Exams of Health sciences

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Gram positive bacteria

Gram positive bacteria

• Cocci: Staphylococcus, Streptococcus, Enterococcus.
• R ods: Bacillus, Corynebacterium,Nocardia, Clostridi um, Actinobacteria and Listeria.
• In Gram-positive bacteria, the S-layer is attached to the peptidoglycan layer.
• Gram +ve bacteria have teichoic acids in the cell wall.
• Gram +ve: retain the crystal violet dye due to a thick layer of peptidoglycan in the cell wall that encases their cell membrane hence retains the stain.
• N.B. In Gram-ve bacteria, the S-layer is directly attached to the outer membrane.

Gram-positive Bacteria Gram-negative Bacteria Gram reaction Retain crystal violet dye and stain dark violet or purple Can be decolourized to accept counter stain (Safranin or Fuchsine); stain red or pink,. Peptidoglycan layer Thick (multilayered)^ Thin (single-layered) Teichoic acids Present in many^ Absent Periplasmic space Absent present Outer membrane Absent^ Present Lipopolysaccharide (LPS) content Virtually none High Lipid and lipoprotein content Low (acid-fast bacteria have lipids linked to peptidoglycan) High (due to presence of outer membrane) Flagellar structure 2 rings in basal body^ 4 rings in basal body Toxins produced Primarily Exotoxins^ Primarily Endotoxins Resistance to physical disruption High Low Inhibition by basic dyes High Low Susceptibility to anionic detergents High Low Resistance to sodium azide High^ Low Resistance to drying High^ Low Cell wall composition Cell wall is 100-120 Armstrong thick, single layered. Lipid content of cell wall is low; Murein content is 70-80% (Higher). The cell wall is 70-120 Armstrong thick, two layered. The lipid content is 20-30% (High), whereas Murein content is 10-20% (Low). Antibiotic Resistance More Susceptible to antibiotics^ More Resistant to antibiotics.

Gram + cocci Catalase test Coagulase + Staph. aureus Coagulase test Coagulase - Staph. epidermitis Catalase + Streptococci Catalase + Staphyllococci

Differentiating Gram positive

Staphylococci

Kingdom: Bacteria Phylum: Firmicutes Class: Bacilli Order: Bacillales Family: Staphylococcaceae Genus: Staphylococcus Species: aureus, epidermidis, saprphyticus , etc

Staphylococci characteristics

• Staphyloccocci - from Greek “stapyle” (bunch of grapes)
• Gram positive cocci (single, in pairs, or irregular grape- like clusters)
• 0.5-1.5μm in diameter
• Non Motile
• Non spore-forming and non flagellated
• Facultative anaerobes
• Hardy organisms surviving many non physiologic conditions .e.g. High salt tolerance (up to 10% NaCl), drying, heat (they withstand 50 °C for 30 minutes)
• Catalase positive (usually)
• Capsule variable - usually negative
• Colonies: round, smooth, raised, and glistening
• Rapidly develop resistance to many antimicrobial agents

Staphylococcus infections

• Staphylococcus aureus: a. boils d. bacterial pneumonia b. abcesses e. food poisoning c. wound infections f. toxic shock syndrome
• S epidermidis: 75% of these infections caused by coagulase-negative staphylococci
• S saprophyticus: common cause of urinary tract infections in young women

11 Coagulase-negative staphylococcus Frequently involved in nosocomial and opportunistic infections

• S. epidermidis – lives on skin and mucous membranes; endocarditis, bacteremia, UTI
• S. hominis – lives around apocrine sweat glands
• S. capitis – live on scalp, face, external ear
• All 3 may cause wound infections by penetrating through broken skin
• S. saprophyticus – infrequently lives on skin, intestine, vagina; UTI
• Others: S. haemolyticus, S. hominis, S. warneri, S. xylosus

A. Staphylococcus aureus

• Major human pathogen
• Habitat - part of normal flora in some humans and animals
• Source of organism - can be infected human host, carrier, fomite or environment

14

Epidemiology and Pathogenesis

• Present in most environments frequented by humans
• Readily isolated from fomites
• Carriage rate for healthy adults is 20-60%
• Carriage is mostly in anterior nares, skin, nasopharynx, intestine
• Predisposition to infection include: poor hygiene and nutrition, tissue injury, preexisting primary infection, diabetes, immunodeficiency
• Increase in community acquired methicillin resistance - MRSA

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Virulence factors of S. aureus

Enzymes :

• Coagulase – coagulates plasma and blood; produced by 97% of human isolates; diagnostic
• H yaluronidase – digests connective tissue
• Staphylokinase – digests blood clots
• DNase – digests DNA
• Lipases – digest oils; enhances colonization on skin
• Penicillinase – inactivates penicillin

17 Virulence factors of S. aureus Toxins :

• Hemolysins (α, β, γ, δ) – lyse red blood cells
• Leukocidin – lyses neutrophils and macrophages
• Enterotoxin – induce gastrointestinal distress
• Exfoliative toxin – separates the epidermis from the dermis
• Toxic shock syndrome toxin (TSST) – induces fever, vomiting, shock, systemic organ damage

19 Separation of Clinically important bacteria

Natural history of disease

• Many neonates, children, adults - intermittently colonised by S. aureus
• Usual sites - skin, nasopharynx, perineum
• Breach in mucosal barriers - can enter underlying tissue
• Characteristic abscesses
• Disease due to toxin production