Gram positive bacteria

• Gram Positives


  • Thick peptidoglycan wall (90%), many sheets of peptidoglycan stacked on top of each other

  Contains surface antigens including teichoic acids and lipoteichoic acids
• Cocci shapes


  • Coccus (single-celled)

  Diplococci (occur in pairs)

  Tetrad (group of 4 in same plane)

  Sarcina (cube-like shape)

  Streptococci (chain-like)

  Staphylococci (grape-like cluster)
• Micrococcus


  Obligate Aerobe

  Catalase positive

  Tolerates salt and drying fairly well

  Often pigmented

  Can be part of the normal skin flora (e.g. Micrococcus luteus)

  Can cause infections in the immunocompromised
• Staphylococcus


  Facultative anaerobe

  Catalase positive

  Tolerates salt and drying fairly well

  Often pigmented (S. aureus: gold/yellow)

  Commonly found on humans and animals (e.g. S. epidermidis)
• Infections caused by Staphylococci

  • Acne, boils, pimples, impetigo

  Pneumonia

  Meningitis

  Many of these diseases are suppurative (pus-forming)

  Common in normal flora and upper respiratory tract

  Serious infections occur when resistance is low
• Staphylococcus aureus and virulence

  Can produce extracellular toxins or enzymes

  Coagulase coagulates fibrin to cause clots (Exoenzyme)

  Leukocidin causes cell destruction and pus formation (Exotoxin)

  Can produce enterotoxins (associated with foodborne illnesses)
• Toxic Shock Syndrome (TSS)

  Extremely serious outcome of a S. aureus infection – rare but life threatening

  High fever, rash, vomiting, diarrhoea and occasionally leads to death

  Associated with tampon use, but rare

  Can occur in men and women following surgery

  Massive inflammatory response, which leads to associated symptoms
• Staphylococcus aureus and Food Poisoning

  S. aureus can produce staphylococcal enterotoxins:

  Superantigens which lead to massive immune response

  Lead to increased permeability of the intestine

  Severe, short-lived diarrhoea and vomiting
• Methicillin Resistant S. aureus (MRSA)


  Hospital 'superbug'

  Associated with the overuse (and incorrect use) of antibiotics

  Selection of resistant strains

  Often seen in patients with lowered resistance

  Others can be carriers

  Drug therapy a problem – vancomycin?
• Streptococcus sp.


  Facultative anaerobe and catalase negative

  Non-motile with fastidious nutritional requirements

  Often part of the normal flora of skin and mucosal membranes

  Some strains are aggressive pathogens, whereas others are opportunistic pathogens
• Streptococcal Diseases: S. pyogenes


  • Streptococcal pharyngitis (strep throat)

  Impetigo

  Scarlet fever

  Necrotising fasciitis

  Rheumatic fever

  Frequent isolate of the URT

  Acute bacterial infection
• Streptococcal Pharyngitis: S. pyogenes


  Severe sore throat

  Enlarged tonsils

  Tonsillar exudate

  Tender cervical lymph nodes

  Mild fever

  General malaise
• Scarlet Fever: S. pyogenes

  Some strains of S. pyogenes carry a lysogenic bacteriophage encoding an exotoxin

  This exotoxin is responsible for most of the symptoms of scarlet fever:

  Pink/red rash

  Damage to blood vessels

  fever
• Streptococcal Impetigo: S. pyogenes


  Causes infection of superficial layers of the skin

  Two types:

  Bullous – large, painless, fluid-filled blisters

  Non-bullous – more contagious, sores which quickly rupture to leave yellow/brown crust

  Also caused by S. aureus
• Necrotising fasciitis: S. pyogenes


  Can also be caused by S. aureus

  'Flesh-eating' bacteria

  Exotoxins A and B recruit T-cells to infected tissues

  1. cells secrete cytokines which activate effector cells

  Leads to massive inflammation, tissue destruction and death in up to 30% of cases
• Streptococcus pneumoniae


  Causes lung infections

  Capsule capable of resisting phagocytosis

  Cells invade alveolar tissue of lower respiratory tract leading to strong inflammatory response

  Can lead to bacteraemia

  Pneumococcal pneumonia

  30% mortality in untreated cases

  1. 10% mortality with antimicrobial treatment

  Vaccine for at risk groups to protect against most common pathogenic strain
• Listeria monocytogenes


  Gram-positive coccobacilli

  Require full aerobic or microaerobic conditions for growth

  Major foodborne illness

  Ready to each foods such as some cheeses and pâtés

  Survives in refrigerators (psychrophile)

  Mild illness to fatal meningitis

  Listeriosis a serious problem in pregnancy
• Gram positive organisms can cause a wide range of infections of varying severity
• Individual Gram positive species can cause a wide-range of diseases: e.g. S. aureus can cause TSS, Impetigo, food poisoning etc)