Microorganisms Microorganisms o Prokaryotic cells no nucleus characterized by simplicity o Photosynthetic autotrophs o Chemosynthetic autotrophs o Heterotrophs Reproduction Plasmids o Reproduced by binary fission splitting into two every 20 30 minutes under optimal conditions o Mechanism simpler than for eukaryotic cells o Simplicity gives potential for very rapid population growth o Circular piece of DNA that carries only a few genes o Replicate independently of the main chromosome o Number of plasmids in a cell remains consistent from generation to generation o F plasmids permit conjunction R plasmids carry resistance genes and carry genes for antibiotic resistance Bacteria o Types Gram positive Gram negative Anaerobic Aerobic Spore forming Non spore forming Heterotrophic Autotrophic Factors that affect growth o Nutrients o pH Some can survive on carbon nitrogen and a trace of few minerals Others need a more complete source of nutrients fastidious Most food poisoning organisms are fastidious Refers to relative acidity of the medium As pH decreases the amount of present acid increases Most microorganisms grow best around a pH of 7 No pathogenic organisms can grow at a pH below 4 6 Some spoilage organisms can grow at low pH values Some are very acid tolerant o Oxygen tension Aerobic organisms require oxygen to grow Anaerobic organisms find oxygen toxic and will die in its Facultative organisms can grow with or without oxygen most presence grow better with o Temperature Psychrophilic grow best around 20 degrees C or below Mesophilic grow best between 20 40 degrees C Thermophilic grow best about 50 degrees C Thermoduric capable of survival above normal pasteurization o Water activity Availability of water to enter into chemical reactions Ranges from 0 1 activity of pure water being 1 Presence of salt sugar or other substances that can bind water will lower the water activity of a substance Some are capable at growing at low water activities others are not In many foods presence of salts and sugars control the growth of undesirable organisms o Presence of other microorganisms Presence of many harmless organisms can slow growth of few harmful ones Some organisms secrete substances into the growth medium that inhibit the growth of other organisms ex Penicillin Bacteriocins produced by some organisms and inhibit the growth of other organisms