Koch's Postulate, Bacterial Population Growth And Gamma Proteobacteria

Koch’s Postulate

For many years, epidemics of contagious infections were believed to be as a result of the wrath of ancient gods, or miasma. German physician Robert Koch (1843-1910) helped in establishing a method useful in identifying a disease-causing agent.  Koch's postulate refers to the four criteria formulated in 1880's to help in the determination of a causal relationship between a disease and its causative microbe (Inglis 2007, p.56). The postulates state that;

1. The pathogen must be present in sufficient amounts in all the host organisms that suffer from the infection but is absent in organisms not suffering from the disease. The infectious microorganism must, therefore, be found to be in direct association with the infection in the organism examined.
   
   
2. Pure cultures of the infectious microorganism previously isolated from the diseased organism must be grown. The pathogen is isolated and grown in pure culture in artificial media.
   
   
3. On introducing the cultured microorganism to a healthy but susceptible organism, it should be able to cause symptoms of the primary disease.
   
   
4. The microorganism is then isolated again from the infected experimental organism and must be same as the primary infectious microorganism. 

Koch applied the postulates to confirm that anthrax, was caused by Bacillus anthracis in cattle, and in human beings, tuberculosis was a disease caused by a different bacteria. Today, scientists follow these basic principles when attempting to establish the causes of infectious diseases (Plowright et al. 2008, p.426). These postulates were developed as basic guidelines useful in identification of pathogens that were causing diseases that were strange to humankind.

Koch’s postulates have several setbacks despite playing a crucial role in the development of the field of microbiology. To give an example, he had a belief that cholera was as a result of microbial infection, but could not fulfill the postulates (Breitschwerdt et al. 2013 p.425). Also, Koch realized that the causative agent of cholera, Vibrio cholera, was confirmed to be available in both infected and non-infected individuals, making his second postulate invalid. Most viruses also do not cause diseases every infected person, a requirement of postulate one. This is for example poliovirus which causes paralytic infection in nearly one percent of their subjects. Further shortcomings of the postulates are that infection with one virus may result in various diseases, while more than one virus can lead to a single disease. Postulates number two and three cannot be justified for viruses that cannot be cultured, or for viruses with no suitable identified animal model. The postulates have also had an influence on molecular examination of many microbial infections by scientists. This led to the development of a molecular type of Koch's postulate to help in guiding identification of microbial genes that encode virulent factors.

The growth cycle of bacterial populations

Under favorable laboratory conditions, bacterial cells can be inoculated into a fresh medium. The bacteria will increase in size and masses then in number. The growth of bacteria is however affected by both nutrients present in the growth medium and physical parameters. Some of the physical parameters include growth temperature, medium pH, osmotic pressure and amount of moisture available for growth. Nutritional parameters include quantities of nitrogen and carbon, phosphorus, Sulphur including trace elements found in the growth medium (Cooper 2012, p.530. Bacterial cells multiply by binary fission in a geometric manner. During studying of bacterial growth population, the bacterial cells with the ability to reproduce are inoculated to a sterilized broth and then incubated ensuring optimal growth conditions are maintained (Huang 2013, p.284). Understanding bacterial growth is better done by plotting a curve of cell growth against growth time as shown below.
 

1. Lag phase. In this phase, there is very little or no bacterial growth taking place. This is because the bacteria are still adjusting to their new environment and synthesizing various metabolic substances. The length of this phase depends on several factors, for example, the ability of an organism to adapt to various environments with different sets of conditions vary with different species of bacteria inoculated. (Hall et al. 2014, p.283).
   
   
2. Exponential or log phase. Here the bacteria have fully adapted and are in a rapidly diving and growing state. If the growth is unlimited, binary fission takes place at a constant rate. The resources are in plenty, therefore, encouraging rapid growth. In pathogenic bacterial species, this is usually the stage where disease symptoms are experienced as the heavily dividing cell leads to greater tissue damage to the host organism.
   
   
3. The stationary or stagnant phase. This is the phase in the growth of bacteria cells where the number of bacteria remains the same since the rate of cell death becomes equal to the rate of cell division. This is because much of the nutrients were consumed during the rapid growth phase and only small amounts left (Linn et al. 2013, p.3170). Toxic byproducts are also being released facilitating the death of some bacterial cells. This, therefore, means that growth cannot continue at the same rate due to insufficient nutrients and accumulation of toxic byproducts.
   
   
4. Death or decline phase. This is the fourth and last phase where there is a steady decline in bacterial population. The bacteria totally loses its ability to divide due to the accumulation of toxic byproducts and nutrient depletion in the media hence facilitating bacteria to enter the death phase. The population of dead cells is more than the amount of live cells. Those cells that are resistant these hard environmental conditions are likely to survive.

The growth cycle of bacterial populations

The gamma proteobacteria consist of a large, diverse group of bacteria that are very important both in medicine and in scientific research purposes. Gamma proteobacteria include a wide variety of pathogens, for example, Escherichia coli, Vibrio cholerae, and many others. Most of them are rods, and all of them are unicellular. This phylum is divided into two major groups such that one group is photoautotrophic for example the purple sulfur bacteria that can use bacteriochlorophylls to help in capturing light energy for photosynthesis and the other group heterotrophic. Particular examples discussed are;

1. Vibrio cholerae

Vibrio cholerae is ‘comma' in shape gram negative that do not form spores usually having a single, polar flagellum useful for locomotion (Mala et al. 2014, p.36). They exist in several strains, some pathogenic which produce the cholera toxin and while non-pathogenic strains do not produce this toxin. Its natural habitat is surface water. Some strains of this bacteria cause the infection cholera.

On infection with Vibrio cholerae patients present with profuse watery diarrhea, abdominal cramps, and occasional vomiting. This lead to dehydration with signs and symptoms such as weakness, sunken eyes, hypotension, thirst among many other symptoms. Progression of these symptoms can lead to death.

Vibrio cholerae thrives in a water environment, especially in surface water. These cholera infections are commonly acquired through drinking water that has been contaminated by Vibrio cholerae from the feces of an infected individual. Transfer of genes is a common phenomenon in many bacteria; therefore, recombinant genes can result in new pathogenic strains which are resistant to current treatment options available.  

2. Escherichia coli

This is a Gram-negative, facultative and a rod in shape bacterium commonly that commonly inhabitat the lower lumen of mammals, and animal feces. Escherichia coli cannot sporulate hence easy to eradicate by simple sterilization or boiling. In a culture medium Escherichia coli preferably grow at 37 degrees Celsius (Croxen 2013, p.830). E. coli can be classified into six groups depending on its virulent characteristics, such as enteroinvasive, enteropathogenic, enterotoxin,  enterohemorrhagic, verotoxigenic and entero- adherent aggressive E. coli. These virulent strains cause several intestinal infections such as gastritis and extraintestinal infections such as mastitis and urinary tract infections. However, most E. coli are usually not harmful to humans or other animals. Most of the E. coli habitat in the intestines where they assist the body in breaking down ingested foods as well as with vitamin K production, waste processing, and food absorption.

Current statistics has shown that a significant number of the population get sick from foodborne diseases every year, most frequently because of consuming undercooked or raw food (Backert et al. 2017, p.21). Foodborne illnesses can easily be prevented by ensuring food safety before consumption. Most common causes of foodborne illnesses are bacteria such as Salmonella, Escherichia coli, and Clostridium botulinum. Some of the safety procedures for preventing foodborne illnesses include;

• Cleanliness- Hand washing should also be done after going to the toilet. Before handling food products, one should ensure his or her hands are thoroughly cleaned and dried.  Ensure the utensils and other food contact surfaces are also cleaned and sanitized. Fresh vegetables and fruits are thoroughly washed in excess water to help in removal of excess agricultural chemicals and any fecal contamination present. Caution is taken not to wash pathogens from meat, fish or poultry as this may only spread the pathogens.
  
  
• Separation of food products should be separated during shopping, when traveling or even at home to ensure cross contamination is avoided. Cross-contamination refers to transfer of pathogenic microorganisms from raw food products e.g. raw pork or dirty people to food meant for human consumption. Cross contamination can be prevented by physically separating food products preventing their juices from dripping onto others. This can be done by placing them in separate carrying bags during shopping. Food can also be stored in separate containers to avoid contact between prepared and raw foods (Balzaretti and Marzano 2017, p.202).  When preparing food ensure that different equipment e.g. knives are used when handling uncooked food.
  
  
• Keep food at their various safe temperatures. Freeze foods accordingly, all cooked and perishable food since cold temperatures inhibit the growth of infectious bacteria. Place a thermometer in the refrigerator and monitor regularly. Perishable foods for example vegetables should be refrigerated immediately after purchase. Cooked food should not be left for more than two hours after cooking at room temperature. Cooked food should be kept piping (more than 60 degrees Celsius) before serving. Avoid storing food for long periods even when using the refrigerator. Also avoid freezing thaw food at room temperatures (Sadilek et al. 2016, p.3985).  
  
  
• Food should be cooked thoroughly for long enough periods that will permit killing of the pathogen. Soups and stews should be boiled until 70 degrees Celsius is reached. After cooking ensure juices are clear for meat and poultry. A thermometer should be ideally used. Cooked food should be reheated thoroughly before consumption.

All these measures if put in place are useful in ensuring that incidences of foodborne diseases are minimized and eventually eliminated.

References

Inglis, T.J., 2007. Principia aetiological: taking causality beyond Koch's postulates. Journal of medical microbiology, 56(11), pp.1419-1422.

Plowright, R.K., Sokolow, S.H., Gorman, M.E., Daszak, P., and Foley, J.E., 2008. Causal inference in disease ecology: investigating ecological drivers of disease emergence. Frontiers in Ecology and the Environment, 6(8), pp.420-429.

 Breitschwerdt, E.B., Linder, K.L., Day, M.J., Maggi, R.G., Chomel, B.B. and Kempf, V.A.J., 2013. Koch's postulates and the pathogenesis of comparative infectious disease causation associated with Bartonella species. Journal of comparative pathology, 148(2), pp.115-125.

Cooper, S., 2012. Bacterial growth and division: biochemistry and regulation of prokaryotic and eukaryotic division cycles. Elsevier.

Huang, L., 2013. Optimization of a new mathematical model for bacterial growth. Food Control, 32(1), pp.283-288.

Hall, B.G., Acar, H., Nandipati, A. and Barlow, M., 2014. Growth rates made easy. Molecular biology and evolution, 31(1), pp.232-238.

Lin, S.Y., Hameed, A., Liu, Y.C., Hsu, Y.H., Lai, W.A. and Young, C.C., 2013. Pseudomonasformosensis sp. Nov., a gamma-proteobacteria isolated from food-waste compost in Taiwan. International journal of systematic and evolutionary microbiology, 63(9), pp.3168-3174.

Mala, E., Oberoi, A. and Alexander, V.S., 2014. Vibrio isolates from cases of acute diarrhea and their antimicrobial susceptibility pattern in a tertiary care hospital. International Journal of Basic and Applied Sciences, 3(1), pp.35-37.

Croxen, M.A., Law, R.J., Scholz, R., Keeney, K.M., Wlodarska, M., and Finlay, B.B., 2013. Recent advances in understanding enteric pathogenic Escherichia coli. Clinical microbiology reviews, 26(4), pp.822-880.

Backert, S., Tegtmeyer, N., Ó’Cróinín, T., Böhm, M. and Heimesaat, M.M., 2017. Campylobacter-features, detection, and prevention of foodborne disease.

Balzaretti, C.M. and Marzano, M.A., 2013. Prevention of travel-related foodborne diseases: Microbiological risk assessment of food handlers and ready-to-eat foods in northern Italy airport restaurants. Food control, 29(1), pp.202-207.

Sadilek, A., Kautz, H.A., DiPrete, L., Labus, B., Portman, E., Teitel, J. and Silenzio, V., 2016, February. Deploying nEmesis: Preventing Foodborne Illness by Data Mining Social Media. In AAAI (pp. 3982-3990).