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Unformatted text preview: Chapter 5 cont Tuesday, February 03, 2009 11:01 AM Begin Exam 2 pH pH- relative hydrogen ion concentration in a solution Scale is from 0 to 14 7 is neutral, < 7 is acidic, > 7 is basic Most bacteria grow at pH of 6-8 Bacteria can be found to exist at almost any pH Most cells internal pH remains near 7 regardless of pH of their environment Most organisms have a pH range at which they can grow of 2-3 pH units Acidity or alkalinity of an environment can greatly affect microbial growth Weak acids can pass through membranes Ex: Good food preservatives Classification based on optimal pH Some organisms have evolved to grow best at low or high pH, but most organisms grow best between pH 6 & 8 and are called neutralophiles (neutrophiles) Acidophiles- grow best at low pH Stability of CM is critical since increases in pH can cause lysis Ex. Many fungi, Thiobacillus produces sulfuric acid, volcanic thermal soil archaea Picrophilus oshimae grows optimally at pH 0.7 Alkalophiles- grow best at high pH found in soda lakes & high carbonate soil Many species of Bacillus live in very alkaline soils Bacillus firmus has a pH range of 7.5-11 Proteases and lipases made by alkaliphiles are mass produced & used in household detergents. Oxygen Microorganisms vary in their need or tolerance of oxygen (O 2 ) & can be grouped based on their requirements for O 2 . oxic environment- O2 is present anoxic environment- no O2 present Aerobes- use O2 to generate energy by respiration Facultative aerobes use O 2 in respiration but can also grow in anoxic environments Ex. Streptococcus mutans on teeth ; E. coli in large intestine Obligate aerobe- use O 2 in respiration & require oxic environments for growth. Grow at atmospheric O2 levels (21%) Ex. Micrococcus luteus Microaerophile- use O 2 in respiration but require low O2 concentrations, 2-10% (microoxic environments) to grow. Ex. Streptococcus pneumonia Anaerobes- cannot use O2 in respiration and may be inhibited or killed by O2 Aerotolerant anaerobes- do not use O 2 to generate energy but can survive in presence of it. Ex. Streptococcus pyogenes Obligate anaerobes- can only grow in anoxic environments, may die if even minute amount of O2 is present Ex. Clostridium sporogenes, Bacteroides in large intestine A reducing agent can be added to a medium to test an organism's requirement for O2 Thioglycolate reacts with O2, reducing it to water In a culture medium, thioglycolate will convert all O 2 to water; only top of culture is exposed to O 2 in the air. Special techniques are needed to grow aerobic & anaerobic microorganisms in the laboratory....
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