Ans- C Q1 Exotoxin production occurs in which phase? Q2 Maximum toxin synthesis in which phase? Answers- Q1. Late phase of Log phase Q2. Stationary phase Growth and Physiology Of Bacteria Bacteria divide by binary fission Nuclear division precedes cell division A. Generation time The time required for a bacterium to give rise to two daughter cells E.coli 20 minutes, M.tuberculosis 20 hours, M. leprae 20 days Total count :(TC) Total number of bacteria present in a specimen irrespective of whether they are living or dead Viable count : (VC) This measures only living cells which are capable of growing and producing a colony on a suitable medium B. Bacterial growth curve Lag phase: Multiplication does not begin immediately, adapt themselves, and increase in size and metabolic activity, duration varies for every bacterium and medium, VC and Tc constant Log phase: Bacteria multiply at their maximal rate, the number increases exponentially, limited duration, VC and TC increase. Exotoxins are produced in this stage Stationary phase: Number of viable cells remains constant, VC constant, TC increases, stage where toxin production occurs Phase of decline: Death rate exceeds the rate of reproduction, VC decreases, TC constant/decreases in case of autolytic bacteria. Extra Edge - Measurements of microbial death Thermal death time- minutes required to kill all cells in a suspension at a given temperature Z-factor / Z value - no. of degrees centigrade required to produce 10 fold change in thermal death time. It is a measure of thermal resistance of spore to a particular process. eg B. sterothermophilus with Z factor of 10-14 is controlled for autoclave. D-value - no. of minutes exposure to produce a log reduction in the number of viable spores at the temperature of the test.