Lab 4: Environmental Influences and Control of Microbial Growth

what do microbiologists do

optimize conditions for growth

what do medical practitioners do

limit growth to prevent disease

what are parameters that influence microbial growth

oxygen, uv light, lysozymes, temperature, and alcohol

obligate aerobes 

must grow in oxygen and use O2 as the final electron acceptor 

examples of obligate aerobes

pseudomonas, micrococcus, and many bacillus

microaerophiles

prefer oxygen concentration of 2-10%, oxygen concentration of atmosphere is 20%

example of microaerophile

helicobacteria pylori

facultative anaerobes

have a flexible metabolism, in the present of oxygen they conduct respiration, and in the absence of oxygen they conduct fermentation

example of facultative anaerobes 

E.coli 

aerotolerant anaerobes

tolerate the presence of O2 but don’t require it, and they strictly use fermentation which is why they are also called obligate fermenters

example of aerotolerant anaerobes

enterococcus faecalis and streptococcus pyogenes

obligate anaerobes 

cannot grow in the presence of oxygen, though tolerance can vary among species, and use inorganic ions as terminal electron acceptor

where are obligate anaerobes found

soil, cattle rumen, and intestines

examples of obligate anaerobes

clostridium and methanococcus

what are reactive oxygen species 

they are produced as respiration by products (ie H2O2) that can damage DNA, proteins, and coenzymes 

what do more microbes have

enzymes that neutralize reactive oxygen species (ROS)

catalase

degrades H2O2 into H2O and O2

superoxide dismutase 

converts superoxide anion to H2O2 and then catalase degrades H2O2

what do most strict anaerobes not have

catalase of superoxide dismutase but they have other methods that deal with ROS

fluid thioglycollate medium

supports growth of aerobic and anaerobic bacteria that contains resazurin dye that turns pink in the presence of O2

FTM test

after inoculating 2 FTM tubes with E.coli and agrobacterium tumafasciens at 37 degrees celsius for 24 hours, it was revealed that agrobacterium tumafascines was an obligate aerobe because there was growth at the top of the tube, and E.coli was an facultative anaerobe because there was growth throughout the tube

what do microbes grow wide over 

temperature ranges 

psychrophiles temperature

-5-15 degrees celsius

mesophiles temperature

25-45 degrees celsius

psychotrophs temperature 

type of mesophile that can grow at 4 degrees celsius, are tolerant of cold temperatures although optimal temperatures are better, however they are a problem in the food industry because they grow at 4 degrees which is temperature of a fridge

thermophiles temperature

45-70 degrees Celsius

hyperthermophiles temperature

>75 degrees Celsius

temperature test 

test the growth of E.coli at 4 degrees, 25 degrees, 37 degrees, and 55 degrees which showed that E.coli should have grown at 37 degrees because that is the human body temperature

uv light

non ionizing radiation with a wavelength of 4-400 nm

what does UV light cause 

pyrimidine dimers in DNA which are covalent bonds between 2 adjacent thymine or cytosine molecules that prevent replication 

SOS system of uv light

removes dimers and replaces pyrimidine molecules because too many dimers can kill cells

what does the effectiveness of UV depend on

exposure time, blocking (plastic, glass, etc), and presence of endospores

B. subtilus vs E.coli 

B. subtilus forms endospores under negative conditions, but in lab they did not grow any because they were incubated for little time 

what are lysozymes

naturally occurring antimicrobial compounds found in tears, saliva, and egg whites

what are most cell walls composed of

complex polymer called peptidoglycan that consists of NAG and NAM subunits

what bond does lysozyme break

the beta 1-4 bond in interlinked NAG and NAM bonds

antiseptics

agents that inhibit/kill microbes but not endospores, and are gentle enough for human skin

how does alcohol kill bacteria 

60-80 percent alcohol kills bacteria by denaturing proteins/membranes 

why did we dip the left thumb twice without alcohol

so we know how much bacteria are removed from the finger by contracting the agar

why did we keep the lids on for some of the UV light plates 

to act as a control so we could see if there is less bacterial growth when the lid is kept on