Bacterial Physiology

Cell Membrane:

• Why is the cell membrane impermeable to hydrophilic molecules? Due to the presence of a hydrophobic barrier.

• Forms of substance transport in bacteria:

• Passive:

• Simple diffusion

• Facilitated diffusion (e.g., glycerol, outer membrane porin)

• Osmosis

• Active:

• Group translocation (e.g., glucose, mannose)

• Energy source for group translocation: PEP (phosphoenolpyruvate)

Classification of bacteria:

• Based on carbon requirements:

• Autotrophs: Utilize inorganic carbon, less pathogenic.

• Heterotrophs: Utilize organic carbon, primarily pathogenic.

• Based on oxygen requirements:

• Obligate aerobes: Require oxygen for respiration.

• Microaerophiles: Can survive in low oxygen concentrations.

• Obligate anaerobes: Cannot survive in an oxygenated environment.

• Facultative anaerobes: Can survive in both oxygenated and non-oxygenated environments (e.g., E. coli).

Environmental conditions:

• Normal pH: Slightly alkaline, 7.2 – 7.6.

• Acidic pH: Vaginal bacteria.

• Alkaline pH: Vibrio cholerae (cholera bacteria) prefers alkaline peptone environments (pH 8.5).

Growth:

• Generation time: Time required for the number of bacteria in a culture to double.

• Generation time of E. coli: 20 minutes.

• Generation time of Mycobacterium tuberculosis (TB bacteria): 20 hours.

Culture media:

• Nutrient media: Provides nutrients for bacterial growth (e.g., peptone).

• Enriched media: Provides a wide range of nutrients (e.g., blood agar, chocolate agar).

• Selective media: Allows the growth of one type of bacteria while inhibiting the growth of others (e.g., Mannitol salt agar, MacConkey agar).

• Differential media: Allows differentiation of bacterial species based on biochemical characteristics (e.g., MacConkey agar).

MacConkey agar:

• What type of media? Both selective and differential media (contains lactose and the pH indicator neutral red).

• Principle of operation: Based on the ability of bacteria to ferment lactose. Bacteria that ferment lactose produce acid, changing the color of the pH indicator.

Growth phases in liquid media:

• Lag phase: Bacteria adapt to the new environment.

• Logarithmic phase: Bacteria reproduce rapidly, the number increases exponentially (10^6).

• Stationary phase: Bacterial growth ceases due to depletion of nutrients or accumulation of toxic substances.

• Death phase: Bacteria die due to lack of nutrients and accumulation of toxins.

Agar:

• What is it? A polysaccharide extracted from seaweed.

• Cultivation in solid media: Forms colonies, millions of cells originating from a single bacterial cell.

Colony classification:

• Based on opacity:

• S-form: Light gray or clear, even edges, evenly convex and shiny.

• M-form: Opaque, slimy, viscous, sticky, more convex than S.

• R-form: May detach as a patch or mass (dry, rough surface).

Disinfection and sterilization:

• Sterilization: Destruction of all types of bacteria, including spores.

• Sterilization methods:

• Moist heat under pressure

• Dry heat

• Gamma radiation

• Chemicals (only used for heat-sensitive materials)

• Disinfection: Elimination of most bacteria, not necessarily killing spores.

• Some disinfectants can kill spores if contact time is prolonged.