Bacterial growth

• bacteria follow a typical growth curve in culture and in patients

• diagram in syllabus: typical growth curve of bacteria in culture media

• time vs. log number of viable organisms is plotted

• lag phase

  • little or no growth occurs

  • bacteria are becoming accustomed to or are adapting to that growth media

  • quite variable in length—hours or days

• log phase

  • bacteria divide by binary division and the number increases exponentially

  • therefore, taking the log of the number of organisms yields a mostly straight line

  • the rate of growth can be highly variable depending on the particular bacteria

  • in the case of S. pneumoniae growing in lab culture or growing unimpeded in the blood of a patient, the division time is 30 minutes to 1 hour

  • if you do the math, one organism can turn into more than a lethal dose of bacteria if the patient is unable to mount a defense

  • so, organisms that grow very rapidly are able to cause very rapid onset of disease if there are inadequate host defenses

  • in contrast, M. tuberculosis, which causes tuberculosis, has a dividing time of 16-20 hours even under favorable conditions

  • therefore, it is much harder for M. tuberculosis to cause an acute, stormy course of disease similar to that seen in case 1

  • clinically, tuberculosis is characterized by a subacute to chronic course

• stationary phase

  • the bacteria can’t keep growing forever because they eventually reach a number of organisms that either exhausts the supply of one or more critical nutrients or builds up a toxic amount of metabolic byproducts

  • this phase is characterized by little or no net growth of the organism

  • bacteria are still metabolizing at this stage and indeed bacteria are both dividing and dying, but the overall number of bacteria doesn’t really change

  • some organisms can’t remain viable for long periods of time in the stationary phase and hence enter the death phase; other organisms can remain in the stationary phase for a long period of time

  • the stationary phase has large consequences for the transmission of bacteria and disease. examples follow:

  • Neisseria meningitidis causes meningitis and Neisseria gonorrhoeae produces gonorrhea. why are these always spread by close person-to-person contact and are never acquired from the environment? it is because Neisseria has an extremely short stationary phase. by the time the organism stops growing in log phase, they are already in the death phase. right after it leaves a nourishing mucus membrane, it starts to die off

  • Salmonella enteritis is reported in 23 people among 3 area hospitals. they all ate at a common salad bar. culture results show that Salmonella is present in the fresh cantaloupe. what does this case predict about the growth curve of Salmonella? well, it had to live on the surface of the cantaloupe for an extended period of time. so Salmonella has a very, very long stationary phase without much of a death phase and therefore can be transmitted by objects (usually food) because it can survive for prolonged periods of time. (the same is true for S. aureus, which can persist for long periods of time without growing)

• death phase

  • this follows the stationary phase

Category: Microbiology Notes