Chapter 6 Bacterial Growth and Cell Division

bacteria replicate by _ which is _

binary fission, asexual reproduction

binary fission

one cell splits into 2 and they are identical

bacterial cell division steps

DNA replication, protein synthesis and elongation of cytoplasm, septum forms and cell divides

microbial growth

number of cells in a population

microbial populations can become

large in a very short time due to short generation time

generation time

the time it takes for a population of bacteria to double in number

calculating bacterial number formula

N0 (# of organisms at time 0) × 2n (n is number of generations)

what does bacterial growth curve show

change in growth rate over time

what are the phases in bacterial growth curve

lag phase, log phase, stationary phase, death phase

lag phase

bacteria is preparing their cell machinery for growth

log phase

growth increases quickly in a straight line

stationary phase

cells stop growing, shut down growth machinery and turn on stress responses

death phase

cells begin to die quickly

lag phase details

little change in cell number, synthesis of enzymes, length of this phase depends on condition of original culture and new medium

log phase details

cellular reproduction is most active, generation time is constant, increase in population

what growth phase is best to preform experiments and use antimicrobial drugs

lag phase

stationary phase details

growth rate slows, number of cells dying = number of new cells, caused by exhaustion of nutrients, accumulation of waste, change in pH

death phase details

number of cells dying is greater than number of new cells, population decreases quickly

microbes exist in

complex, multispecies communities

for lab studies, microbes must be grown in

pure culture as a single species

solid vs liquid bacterial media

solid is useful to separate mixtures of different organisms, liquid allows bacteria to move freely

bacteria culture media requires

an organism to be viable (able to replicate and form a colony)

solid media

bacterial cells form colonies on media with agar added to create a firm surface

what streaking is used on solid media

dilution/isolation streaking to create an isolated colony

confluent growth

continuous growth that covers the entire surface

serial dilution

steps used to decrease number of colonies of one sample from liquid to solid media

serial dilution advantage

measures the number of viable cells

how many colonies does a plate need to contain to be accurate in serial dilution

between 30 and 300

what do we need to assume in serial dilution

that each live cell divides to produce a single colony, reported as colony forming units (CFU)

pour plate method

correct amount of sample is aseptically transferred onto sterile petri dish, warm molten agar (50 degrees C) is poured on top and swirled into sample

pour plate method disadvantages

heat sensitive microorganisms can be damaged, colonies that form inside agar are smaller and less obvious

spread plate method

correct amount of sample is aseptically transferred to surface of agar plate, inoculum is spread over surface of agar

how to determine CFUs

multiply the starting amount and each level by 10 (10×10, 100×10) and TNTC is too numerous to count

CFU exponents

10^X is the number of times you multiplied by 10 (10^5)

plated growth methods only count

live/viable cells

how to determine the total number of bacterial cells (dead or alive)

direct and indirect cell counts

direct cell counts are determined by

microscopy

indirect cell counts are determined by

fluorescence-activated cell sorter or by measuring turbidity with a spectrophotometer

flow cytometry or fluorescent-activated cell sorting (FACS)

bacteria are placed in a fluid and flowed through the instrument while the instrument uses lasers to detect the bacteria

what does FACS do

counts and separates cells that have different fluorescent properties

what does flow cytometry do

only counts cells and doesn’t separate them

FACS vs flow cytometry output

FACS stacks cells on top of each other, flow cytometry separates the cells

optical density/turbidity

as bacteria multiply, the media turns bloody (turbid), a spectrophotometer is used to measure optical density and estimate number of cells

what wavelength does a spectrophotometer use

600nm

optical density plot

determines turbidity of different concentrations and the number of viable organisms per ml, then a standard curve is drawn to match turbidity/optical density to a specific number of organisms

direct microscopic count

a measured volume of bacteria suspension is placed on a microscope slide to count fields, average/volume (ml)  = number of bacteria/ml

pros and cons of direct microscopic count

good if there’s no time considerations because of no incubation, hard to count motile bacteria, dead cells can be counted as live, high cell concentration is required