Microbial Physiology & Growth Control

Physiology

life processes that allow organisms to survive, grow, and reproduce

Microbial physiology helps explain

where microbes grow and why some die under certain conditions

Genetics help explain

changes in virulence and antimicrobial resistance and can support laboratory identification

Microbial nutritional requirements

Microbes need chemical building blocks for growth and reproduction; nutritional requirements vary by species; some microbes specific growth factors they cannot synthesize

Energy source

Phototroph - light

chemotroph - chemicals

Photograph

source - light

examples - algae, cyanobacteria

Chemotroph

Source - chemicals

examples - most human pathogens

Carbon sources

autotroph and heterotroph

Autotroph

carbon dioxide

examples - environmental producers

Heterotroph

organic compounds

examples - animals, fungi, protozoa

Metabolism

all chemical reactions in a cell

Enzymes

speed up and regulate specific metabolic reactions; each enzyme acts on a specific substrate

Catabolism

large molecules are broken down - releases energy

Anabolism

small molecules are built into larger molecules - requires energy

ATP The Cell’s Short-Term Energy Currency

catabolic reactions release energy and some of that energy is captured in ATP; cells use ATP for growth, transport, movement, and biosynthesis

Aerobic respiration

oxygen used as final electron acceptor and yields higher ATP

Fermentation

does not require oxygen and yields lower ATP

Fermentation without oxygen

it does not require oxygen; begins with glycolysis and produces less ATP than aerobic respiration

produces characteristic end products that can aid laboratory identification or have industrial uses

Why bacterial genetics matter

genotype, phenotype, mutation, plasmid

Genotype

genetic makeup of an organism

phenotype

observable trait

mutation

change in DNA that may be inherited by daughter cells

plasmid

small extrachromosomal DNA molecule

Possible outcomes of mutations

beneficial - improves survival under certain conditions

harmful/lethal - reduces survival or kills the organisms

silent - no noticeable effect on phenotype

Mutation

the bacterium’s own DNA changes

Lysogenic conversion

a temperate phage introduces phage genes that change bacterial traits

phage genes may change the bacterium’s phenotype

some bacterial toxins are associated with phage-acquired genes

Transduction: phage-mediated gene transfer

a bacteriophage accidentally carries bacterial DNA from one bacterium

phage transfers that DNA to another bacterium, which may acquire a new trait

Transformation: uptake of DNA

free “naked” DNA is released into the environment

bacterium takes up the DNA

acquired DNA may become part of the bacterium’s genetic material and change its traits

Conjugation: cell-to-cell transfer

a donor bacterium makes direct contact with a recipient bacterium

DNA, often a plasmid, transfers from one cell to the other

plasmids may carry antimicrobial-resistance genes

Factors that affect microbial growth

nutrients, moisture, temperature, pH, osmotic pressure/salinity, atmosphere

Temperature & Microbial growth

incubation conditions affect recovery

psychrophiles, psychrotrophs, mesophiles, thermophiles

Psychrophiles

cold temp

environmental organisms

psychrotrophs

refrigerator temp

food spoilage risk

mesophiles

moderate; includes body temp

many human pathogens

thermophiles

high temp

hot springs and heat-tolerant environments

pH & microbial growth

pH affects enzyme function and therefore microbial growth

many medically important bacteria grow best near neutral pH

acidophiles prefer acidic conditions; alkaliphiles prefer alkaline conditions

Moisture and desiccation

water is required for metabolism and growth

drying generally inhibits microbial growth and reproduction but may not kill all microbes

bacterial endospores and protozoan cysts can survive drying better than active cells

osmotic pressure & salinity

water moves across membranes toward higher solute concentration

hypertonic environments can pull water out of cells

plasmolysis

Plasmolysis

cytoplasm shrinks away from the cell wall in a hypertonic environment

Encouraging growth in vitro - culture media

culture media provides nutrients and conditions that support growth

artificial media can be liquid, semisolid, or solid

different media help labs grow, isolate, or identify organisms

Different types of culture media

enriched, selective, differential, selective & differential

Enriched

supports fastidious organisms; provides extra nutrients or growth factors

Selective

favors some organisms; inhibits others

controls which organisms grow

Differential

distinguishes organisms that grow

shows visible differences

Selective & differential

does both favor/inhibit and distinguishes organisms

Population growth curve

lag phase, log phase, stationary phase, death phase

Lag phase

cells adjust to the environment

Log phase

rapid exponential growth

Stationary phase

growth balances cell death

Death phase

cells die faster than they divide

Ways to inhibit growth in vitro

sterilization, disinfection, antisepsis, sanitization, aseptic technique

Sterilization

destroy/remove all microbial life; including bacterial endospores

Disinfection

reduce or eliminate pathogens on inanimate objects

Antisepsis

reduce microbes on living tissue

Aseptic technique

prevent contamination and maintain asepsis

Sanitization

reduce microbial counts on public-health standards

Heat physical methods

heat effectiveness depends on temperature and exposure time

autoclaving & pasteurization

Autoclaving

steam under pressure → sterilization

Pasteurization

reduces microbial load → does not sterilize

Cold

slows microbial metabolism and growth

desiccation

inhibits growth while dry; some microbes survive

radiation

damages DNA or other cellular components

filtration

physically removes microbes from fluids or air

Effectiveness of antimicrobial procedures depends on

time, temperature, concentration, type/#/presence of spores of microbes, presence of proteins in feces, blood, vomitus, pus

Growth-control terms

-cidal & -static

-cidal

kills the target microbe; bactericidal, sporicidal, fungicidal, viricidal

-static

inhibits growth or reproduction; bacteriostatic, microbistatic

Chemotherapy

use of chemical/drug to treat disease

Antimicrobial agent

medication used to treat an infection

Antibiotic

antibacterial drug used to treat bacterial infections

antibacterial/antifungal/antiviral/antiprotozoal

named for the microbial target

Goal of Antimicrobial Therapy

target the pathogen while minimizing harm to human cells

reach the site of infection

remain at an effective concentration long enough to work

target structures or processes that differ from human cells

Selective toxicity

greater when antimicrobial targets structures or processes that differ from human cells

Cell wall synthesis

weakens or prevents bacterial cell-wall formation

cell membranes

damages membrane integrity

Nucleic acid synthesis

blocks DNA/RNA production

Protein synthesis

blocks ribosome function

Enzyme activity

blocks essential metabolic steps

Narrow-spectrum antibiotic

targets a smaller group of bacteria

broad-spectrum antibiotic

targets a wider range of bacteria

Intrinsic resistance

an inherent characteristic of a microbial species

Acquired resistance

develops through mutation or gene acquisition

Antimicrobial exposure creates…

selective pressure that favors resistant organisms

Selection for Resistant Organisms

susceptible organisms are killed or inhibited

resistant organisms survive and may multiply

repeated or unnecessary exposure increases selective pressure

Altered drug-binding site

drug no longer fits the target

reduced permeability

drug cannot enter effectively

drug-inactivating enzyme

drug is destroyed or modified

Efflux pump

drug is pumped out of the cell

New gene acquired

resistance trait spreads through gene transfer

B-Lactamases

Example of a drug-inactivating enzyme - penicillin, cephalosporins, carbapenems

Empiric Therapy

treatment may begin before the exact pathogen is known when delay could be harmful

initial therapy is based on the likely pathogen and clinical situation

Susceptibility testing

lab testing that determines which antimicrobial agents inhibit the patient’s isolate

Undesirable effects of antimicrobials

allergic reactions and drug toxicities can occur

broad-spectrum agents can disrupt indigenous microbiota

killing susceptible organisms may allow resistant organisms to predominate

superinfection can occur when normal microbiota are disrupted

Antimicrobial Stewardship

support appropriate specimen collection before antibiotics when clinically feasible

administer antimicrobials on time and monitor for therapeutic response

assess allergies, adverse effects, and signs of superinfection

educate patients that antibiotics do not treat viral infections

promote infection prevention to reduce the need for antimicrobials