Physical & chemical Factors Microbial growth Physical & chemical Factors

Physical factors: - Temperature - pH - osmotic pressure - radiation. Chemical factors : - Oxygen - carbon

Physical factors :Temperature Temperature is an important factor that determines the rate of growth, multiplication, survival, and death of all living organisms. High temperatures damage microbes by denaturing enzymes, transport carriers, and other proteins. Microbial membrane are disrupted by temperature extremes. At very low temperatures membranes also solidify and enzymes also do not function properly.

Types of temperature Maximum growth temperature - The highest temperature at which growth occurs Optimum growth temperature - The temperature at which the most rapid rate of multiplication occurs. Minimum growth temperature - The lowest temperature at which organisms grow

Psychrophilic organisms: - are capable of growth and reproduction in cold temperatures - Temperature range: −20°C to +10°C. Mesophile : - grow best at 37c Thermophilic - heat-loving microorganism - ~ 50 to 100 c Hyperthermophilic > 100

pH

Acidophilic : - grow at low pH Neutrophilic : - grow at neutral pH ~7 Alkalophilic: - grow at high pH

pH Yeast and molds can tolerate low pH that’s why SDA( pH ~ 5.6 ) medium is considered a selective factor to allow fungi to grow and inhibit bacterial growth. sabouraud dextrose agar SDA

Osmotic pressure Osmotic pressure is the minimum pressure which needs to be applied to a solution to prevent the inward flow of water across a PM. Types of solution: 1. Hypotonic 2. Isotonic 3. Hypertonic

Classification of bacteria according to osmotic pressure 1. Osmotolerant ( salt-tolerant) are those microorganisms which can grow at relatively high salt concentration. 2. Halophiles- Grow in the presence of high salt concentration Normal saline : 0.9% NACL ( 9 g/L)

Radiation UV Ionizing radiation - causes mutations

Chemical Mannitol salt agar High salt conc. Mannitol as carbohydrate source S.aureus gives yellow colonies

Chemical Neisseria spp. : Carbohydrate Utilization test N. Gonorrhea N. Meningitides Glucose Maltose Lactose N. Gonorrhea + - N. Meningitides

Oxygen

oxygen Obligate aerobes: Obligate anaerobes: Facultative anaerobes: They have absolute requirement for oxygen in order to grow. Ex. MTB Obligate anaerobes:   These bacteria grow in the absence of oxygen and for which oxygen is toxic. Ex. Clostridium. Facultative anaerobes:   They are versatile organisms,  capable of growth under both aerobic and anaerobic conditions. They preferentially use oxygen as terminal electron acceptor. e.g.S. aureus Microaerophilic: need only 5-10% O2 ex. campylobacter

Carbon requirement Capnophlic bacteria: - require increased concentration of carbon dioxide (5% to 10%) and approximately 15% oxygen. Air contains 21% oxygen and small amount of (0.03%) of carbon dioxide

Change in these factors affect the growth rate of microbes