Microbial growth and biofilm chapter 6

Microbial growth

An increase in the number of cells

Binary fission

A process by which bacteria reproduce asexually

Steps of binary fission

1. Cell elongates and DNA is replicated 2. New membrane and cell wall form 3. Cross wall forms 4. Cells separate

Physical factors affecting bacterial growth

Temperature

Psychrophiles temperature range

Optimum at 15°C

can grow at 0°C.

Psychrotrophs

Bacteria that cause food spoilage in the fridge (optimum 20-30°C).

Mesophiles

Bacteria that grow best at human body temperature.

Thermophiles

Heat-loving microbes that thrive at 50-60°C.

Hyperthermophiles

Extreme heat-loving microbes that thrive at temperatures greater than 80°C.

Preferred pH range for most bacteria

6.5 to 7.5.

Acidophiles

Microbes that grow in acidic conditions (pH 0.1-5).

Alkaliphiles

Microbes that grow in basic conditions (pH 8.5-11.5).

Halophiles

Salt-loving microbes.

Obligate halophiles

Microbes that require high salt concentrations (e.g.

Facultative halophiles

Microbes that tolerate both normal and high salt conditions.

Elements needed for bacterial growth

Carbon

Chemoheterotrophs

Microbes that use organic molecules as energy sources.

Chemoautotrophs

Microbes that use CO₂ from the atmosphere.

Obligate aerobes

Microbes that require oxygen to grow.

Facultative anaerobes

Microbes that can grow with or without oxygen.

Obligate anaerobes

Microbes that cannot grow or survive in the presence of oxygen.

Aerotolerant anaerobes

Microbes that do not use oxygen but can tolerate its presence.

Microaerophiles

Microbes that require low levels of oxygen.

Culture media

A nutrient-rich substance used to grow microbes in the lab.

Purposes of culture media

Diagnosing diseases

Types of special media

Selective

Selective media

Media that suppresses unwanted microbes and encourages desired ones.

Differential media

Media that distinguishes microbes based on physical/chemical changes (e.g.

Enrichment media

Media that increases the number of desired microbes when they're in small quantities.

Ways to preserve microbial cultures long-term

Deep-freezing (−50°C to −95°C)

Biofilm

A microbial community that forms a slimy layer on a surface.

Reason bacteria form biofilms

As a stress response for protection and survival.

Locations where biofilms can form

On surgical implants

Quorum sensing

Bacterial communication based on cell density that adjusts gene expression.

Problems caused by biofilms in healthcare

They cause 70% of infections and are 1000x more resistant to microbicides.

Benefits of biofilms for bacteria

They share nutrients

Structure of a mature biofilm

Mixed microbial communities including bacteria

Effect of quorum sensing on gene expression

It allows bacteria to sense population density and trigger coordinated changes like toxin or enzyme production.