Microbes - General

Gram Positive

 

Gram Positive cocci

 

Staphylococcus epidermis / aureus 

- 1/2 - 2/3 all infections

 

Streptococcus viridans

- 25%

 

Other

- Enterococci

- Strept. pyogenes / pneumoniae

- Neiserria

 

Gram positive bacilli (all anaerobes)

 

Clostridiae (bacilli)

Listerius

Bacillus

 

Gram Negative

 

Gram negative bacilli

 

10-20%

 

Coliforms

- E Coli, Proteus, Salmonella, enterobacter, acinetobacter

 

Pseudomonas

 

Hemophilus

 

Gram Negative Cocci

 

Neiserria

 

Gram negative Anaerobes

 

Bacteroides

 

Resistant microbes

 

MRSA, MRSE & VRE

 

Cell Structure

 

Cytoplasmic Membrane

 

Innermost layer

- present in both Gram positive and negative

- functions as a permeability barrier and transport system

 

Cell Wall

 

Function

- maintains cell shape

- protects high internal osmotic pressure

- cell wall injury = Lysis

 

Gram positive

- thick Peptidoglycan Layer (x5)

 

Gram negative

- thin Peptidoglycan Layer

- second outer membrane of lipopolysaccharide (endotoxin)

 

Capsule & Glycocalyx

 

Extracellular Polymer

- if forms condensed, well-defined layer, called Capsule

- if forms loose meshwork of fibrils, called Glycocalyx

 

Slime

- polysaccharide Glycocalyx

- envelopes bugs infecting prosthesis

- S aureus & epidermis

- Pseudomonas

 

Function

- adhere to and survive on synthetic surfaces

- protects from host-defense factors / Complement fixation / Neutrophil ingestion

- 500 x more resistant

 

"Window of Opportunity"

- theoretical period when can kill microbe before biofilm forms

- basis behind Acute Early TJR infection regimes

- time frame unknown ? 2/52

 

Spores

- gram positive rods

- Bacillus & Clostridium

- occurs in unfavourable conditions

- resistant to drying / antiseptic

- only killed by Betadine

- killing of spores is the difference between Sterilization & Disinfection

 

Staining

 

1.  Gram Stain

 

Difference is in cell wall

- reason unclear

 

Technique

- flood with Crystal Violet and wash

- flood with Iodine applied and wash

- all microbes are blue at this point

- carefully decolourise with ethanol

- counterstain with safranin

 

Gram positive

- retain crystal Violet - Iodine complex

- purple

- don't take up counterstain

 

Gram negative

- cells completely decolourised

- take up safranin

- take on contrasting Red

 

2.  Acid - Fast Stain / Ziehl-Neilsen

- AFB retain Carbol-Fuschin stain even when decolourised with acid-alcohol

- most commonly Mycobacterium

 

Process

- red Carbol-fuschin applied

- heated on steam bath

- decolourised with hydrochloric acid in alcohol

- contrasting blue counterstain applied

 

Acid-fast bacteria appear red

- others are blue

 

Antimicrobial Actions

 

Four basic methods of action

 

1.  Cell Wall Synthesis

- ß lactam drugs 

- Vancomycin

 

2.  Inhibition of Cell Membrane Permeability

- Polymyxins

- Amphotericin B & Fungi

 

3.  Inhibition of Protein Synthesis

- Gentamicin

- Erythromycin

- Tetracyclines

 

4.  Inhibition of Nucleic Acid Synthesis

- Quinolones (Ciprofloxacin)

- Rifampicin

- Sulfonamides

- Trimethoprim

 

Antibiotic Resistance

 

1. Genetic Exchange

- plasmids

- Entire chromosomes

2. mutation

 

Mechanisms

- degrade antibiotics / B Lactamases

- modify receptor sites or target

- alter 30s binding site of ribosome

- decrease bacteria's permeability to the antibiotic

- protective glycocalyx

- produce cell membrane antibiotic pumps