Gram Positive and Negative Flashcards
Describe the prokaryotic cell structure
Non-compartmentalised
DNA in cytoplasm rather than within membrane-bound nucleus
How do bacteria reproduce?
- Binary fission
- Spore formation
What are the components of the cell membrane?
Cytoplasm
Cell membrane
Peptidoglycan
Lipopolysaccharide
What is the process of a gram stain?
- Fixation with heat
- Crystal violet (peptidoglycan layer)
- Iodine treatment
- Decolourisation (acetone)
- Counter stain safranin
Describe a gram-positive gram stain result
- Stain blue-black
- Thick peptidoglycan
- Some have spores
- Mostly sensitive to penicillin, vancomycin
Describe a gram-negative gram stain result
- Stain pink
- Thin peptidoglycan
- Lipopolysaccharide (=endotoxin)
- Mostly resistant to penicillin, vancomycin
What are the types of aerobic gram positive bacteria?
-Coccus
=Staphylococcus (grape-like, divide in 3D)
=Streptococcus (chains, horizontal division)
-Bacillus
=Bacillus (thick and large)
=Listeria (small and single-celled)
=Corynebacteria (Chinese lanterns)
Examples of Staphylococcus
- S. aureus
- Coagulase negative staph
Describe S. aureus infections
- Includes MRSA
- Is coagulase (virulent factor) positive
- Pus-forming infections
- Blood-stream infections, often device-related- 20-30% mortality
- Toxin illness (scaled skin syndrome)
Describe coagulase-negative staphylococci infections
- Lots of different species (staph epidermis, S, capitis)
- Normal commensal flora of skin and mucosal surfaces, help skin integrity and compete with pathogenic bacteria
- Normally only pathogenic with foreign body (prosthetic joint)
Examples of Streptococcus
-beta haemolytic (complete destruction of haemoglobin, highly virulent)
=(Lancefield groups A,B,C,F,G= antigens in cell walls and membranes, agglutinates with reciprocal serum)
-alpha haemolytic (incomplete)
=Strep pneumoniae
=Viridans streptococci
Describe streptococcus pneumoniae
- Alias “pneumococcus”
- CAP (community acquired pneumonia)
- Meningitis
- Bronchitis in COPD
Describe S. viridans
- Mucosal (often mouth) flora
- S. oralis, s. salivarius, s. sanguinis
- Stick to gums, teeth etc, stick to heart valves so cause endocarditis (+primary heart problem)
Examples of group A b-haemolytic strep
- S. pyogenes
- Tonsillitis
- Cellulitis
- Severe necrotising soft tissue infections (necrotising fasciitis)
- Puerperal sepsis (perinatal)
-Group C and G strep- similar to A but less aggressive
Examples of group B b-haemolytic strep
- S. agalactiae
- Normal rectal and vaginal flora
- Neonatal meningitis and sepsis, bacteraemia, pneumonia
- Common cause of infection during perinatal period- ascending through birth canal
- Occasionally a pathogen in adults especially if immunocompromised
Describe Enterococcus
- Gram positive cocci in chains (like strep)
- Normal commensal gut flora
- More resistant to antibiotics (vancomycin resistant enterococci)
- Involved in intra-abdominal sepsis, urinary tract infection, bacteraemia
What are the two main types of Bacillus species?
- bacillus anthracis
- bacillus cereus
Describe bacillus anthracis
- Disease of herbivores, human is incidental host
- Reservoir= soil
- Multiple toxins and virulence factors- oedema toxin most pathogenic
- Humans infected by cutaneous inoculation (IVDU) or inhalation (severe necrotising pneumonia)
- Scottish outbreak 2012 IVDU, notifiable to public health
Describe bacillus cereus
- Food poisoning- vomiting
- Occasional pathogen if infected into sterile sites (endophthalmitis in eye)
- Notifiable to public health
Describe listeria monocytogenes
- Intrauterine/ neonatal septicaemia/ meningitis
- Can multiple at 4C
- Risk in pregnancy from foods like soft cheese and coleslaw
- Notifiable to public health
Describe Corynebacterium diphtheriae
- Transmission: respiratory droplets
- Bacteria in pseudomembrane in throat (potential suffocation)- bull neck presentation due to inflammation of neck fascia
- Diphtheria toxin produced in throat, via bloodstream and then inhibits protein synthesis in heart (so cardiac failure- presents septic but bradycardia) and peripheral nerve
- Vaccine prevention, antitoxin and antibiotics
- Notifiable to public health
What are the gram positive anaerobes?
- Coccus= anaerobic coccus
- Bacillus= clostridia
- Majority sensitive to metronidazole
Examples of medically important Clostridia
- Clostridium tetani
- Botulinum
- Difficile
- Perfringens
Describe C.tetani
- Transmission: spores ubiquitous in soil, then contaminate wounds especially if devitalised tissue
- Umbilicus cut with not sterile eqiupment
- Toxin-mediated disease: Tetanus neuro-toxin causes excitation of motor neurones by blocking release of inhibitory GABA
What are the symptoms and treatment of tetanus?
-Symptoms
=Spasms/ rigidity of voluntary muscle (lock jaw)
=Opisthotonus
=Autonomic system dysfunction, convulsions, hyperextension of neck
-Vaccine against toxin
-Assessment of tetanus prone wounds -use of immunoglobulin
-Slow-growing toxin (2-3 weeks)
-Public health
Describe botulism
- Usually food-borne: spores germinate anaerobically
- Toxin-mediated: neuro-toxin prevents release of acetylcholine
- Symptoms: symmetrical flaccid paralysis- descending (death through respiratory failure)
- Therapeutically BoTox
- Standards of food preparation
- Public health
Describe C. difficile
- Major cause of healthcare associated infection
- Transmission: ingestion of spores (hospital)- survive alcohol
Describe C. perfringens
- Necrotising fasciitis
- Transmission: spores ubiquitous in soil and human gut
- Toxin-mediated disease: main toxin is a toxin, lecithinase (damages cell membranes inc haemolysis)
- Public health
Examples of gram negative cocci
- Neisseria gonorrhoeae
- Neisseria meningitidis
- Moraxella cattheralis (COPD exacerbations)
- Commensal Neisseria in mouth
Describe Neisseria
-Pus in gonorrhoeae =Most common STI =Resistant to antibiotics =Neutrophils -Rash in meningococcal disease =Significant sepsis and shock =Purpuric and non blanching =Public health
Groups of aerobic gram negative bacilli
-Non-fastidious
=sugar fermenters (Enterobacteriaceae)
=Non sugar fermenters (oxidase positive curved and non-curved)
-Fastidious (fussy slow growers)
Examples of Enterobacteriaceae
- E-coli
- Klebsiella
- Salmonella
- Shigella
Describe E. Coli
- Normal human gut flora (most common)
- Most common cause of UTI (ascending)
- Some strain cause diarrhoea (0157)= haemolytic uraemic syndrome
- Common cause of blood-stream infection (bacteraemia) secondary to UTI and intra-abdominal sepsis
Describe Klebsiella
- Second commonest coliform causing UTI and bacteraemia
- Often antibiotic resistant amoxicillin, carbapenemase (acquiring enzymes involved in resistant)
- Spreads easily between patients
- Cousins= enterobacter, proteus, serratia
Describe shigella
- Important cause of gastroenteritis
- Shigella= human reservoir only (colitis)
- Ver infectious, public health
Describe salmonella
- Many hosts
- typhi=typhoid, highly invasive, low and middle income countries
- typhimurium and enteritidis= gastroenteritis
Describe Pseudomonas aeruginosa
- Non-fastidious, non-sugar fermenter oxidase positive non-curved
- Most important non-fermenter
- Reservoir: environment (especially water)
- Human respiratory pathogen where biofilms important (CF, ventilator associated pneumonia in ITU)
- Bacteraemia in immunocompromised (leukaemia)
- Common coloniser in leg ulcers (biofilm)
- Resistant to antibiotics
Describe Vibrio cholerae
-Curved non-fastidious
-Reservoir warm salt water
-Toxin-mediated disease: increased cAMP within cells, decreased Na uptake and increase Cl secretion so massive water loss (rice water stool)
-Death from dehydration and electrolyte imbalance
-Treat with oral re-hydration
Public health
Describe Campylobacter
- Curved non-fastidious
- Commonest bacterial cause of gastroenteritis in UK
- Natural host= birds (80% chickens)
- Self-limiting illness
- Can become invasive during immunosuppression (mortality 40%)
Describe fastidious gram negative bacilli testing
-Difficult to grow in culture
-Need enriched agar plates to grow
=Chocolate (lysed blood)/ specially prepared legionella agar
-May require molecular testing (PCR) to ascertain diagnosis
Examples of fastidious GNB
- Haemophilus influenzae
- Legionella
- Helicobacter
Describe haemophilus influenzae
- Unencapsulated strains are common (respiratory tract COPD, pneumonia in smokers)- chocolate agar plates
- Capsulated serotype b causes meningitis in young children (rare UK as vaccine)
Describe Legionella
- 1976 outbreak of pneumonia US
- Causes severe pneumonia (multi-lobar, hyponatremia, progresses quickly)
- Difficult to grow in lab as slow-growth on enriched agar, urinary antigen or PCR
- Lives in natural and engineered water systems inside amoebae
Describe Helicobacter
- Discovered 1984
- Produces urase= breaks down urea to ammonia, buffers stomach acid to allow survival in anthrum of stomach
- Causes inflammation leading to ulceration
- Oral antibiotics, beta lactam and amoxicillin
Examples and description of GN anaerobes
-Bacteroides
=Associated with intraabdominal and skin and soft tissue infections below waist
=Resistant to penicillin so metronazidole
-Fusobacterium
=Long filamentous rods
=Head and neck infections including brain abscesses
=Mixed intraabdominal infections
=Perirectal abscesses
