When you go to a hospital, you typically expect it to make you better, not get you sicker. Unfortunately, hospitals are filled with bacteria, many of which ‘prey’ on patients with weakened immune systems and preexisting health issues. One such bacterium is klebsiella, a genus of Enterobacteriaceae. This bacteria is estimated to cause nearly 8% of all nosocomial (hospital-acquired) infections each year (Podschun, 1998). Klebsiella pneumoniae is normally found growing in places such as the mouth and on skin, but it can create health issues if it enters the lungs, causing inflammation, hemorrhaging, and necrosis of lung tissue. It is especially dangerous if bacteria have developed antibiotic resistance, as an increasing number of hospital-acquired infections have done.
This resistance is coded for in plasmids (small, circular, transferrable bits of DNA) that the bacteria pick up. A common protein that causes antibiotic resistance is beta-lactamase. This enzyme, breaks open the beta-lactam ring in antibiotic molecules containing this structure, thereby deactivating them. Penicillin is one such beta-lactam-based antibiotic. It is widely used to treat bacterial infections, and resistant bacterial strains are a large reason for concern. While there are other antibiotics that can be used, some of these bacteria have extended-spectrum beta-lactamase genes, which makes them even more efficient at denaturing a wider range of beta-lactam antibiotics. In poorer areas, where less common antibiotics are harder to come by, this is even more of an issue. In 2003, over half of the antibiotics in use were beta-lactam compounds (Elander, 2003).
These infections add time to hospital stays. Not only is this an inconvenience, but it is also an economic drain on the hospitalized individual. These types of infection are also prominent in intensive care units, where patients are already suffering from depressed immune systems, and an added infection without the help of antibiotics can kill a patient.
For this reason, the search for alternate antimicrobials is a high priority. Bacteriophages (viruses that only infect bacteria) hold great promise in this area, but to be effective, they must be able to infect a wide host of bacteria. There are four types of host resistance mechanisms to bacteriophage infection: adsorption inhibition, blocking of DNA injection, restriction-modification, and abortive infection (Weinbauer, 2004). The host restriction system is one of the best-studied parts of this system. In “Characterizing the biology of novel lytic bacteriophages infecting multidrug resistant Klebsiella pneumoniae,” Kesik-Szeloch et al. culture and screen a number of bacteriophages for the pathogenicity in Klebsiella, and their abilities to resist host restriction-modification mechanisms.
The viruses were isolated from 8 water samples collected in Poland, and grown on 11 extended-spectrum beta-lactase producing strains of Klebsiella pneumonia. The bacteriophages were classified morphologically by electron microscopy. All 32 of the lytic phages that were propagated were in the order Caudovirales, which are tailed bacteriophages. Specifically, they were classified into the families Myoviridae, Siphoviridae, and Podoviridae. As previously stated, for a bacteriophage to be a good antibiotic agent, it must kill a large percent of the intended bacteria and also be able to infect a fair range of bacteria. Most of the bacteriophages were found to be fairly ineffective at lysing bacteria outside of the species (K. pneumoniae) that they had been propagated in, but the Siphoviridae and Podoviridae lysed them with a 7-15% efficiency. However, Myoviridae were able to lyse a close relative (K. oxytoca) with 7-37% efficiency. The burst size was then examined in representative phages from each of these families along with their durability to factors such as temperature, chloroform, and pH conditions. Of those tested, Siphoviridae had the largest burst size and was fairly temperature resistant. Most phages were unaffected by chloroform and were stable in a pH range from 5-8, well within the limits of the human body.
One of the main natural defenses of bacteria against viruses is cutting up foreign DNA (with endonucleases), thereby deactivating particles such as viruses. These enzymes cut at specific known DNA sequences. To test how well the selected viruses could withstand bacterial restriction mechanisms, they were treated with an array of endonucleases, and then they were analyzed to see if they were actually cut at all of the predicted cut sites. While some of the viruses were easily digested by the endonucleases, many were rather resistant. Computer models often predicted hundreds of cut sites, none or few of which were observed. The viruses that can resist these mechanisms have the potential to better infect bacterial cells, and would probably be good candidates to further characterization.
Overall, this experiment may not have found the next cure for bacterial infections, but screens such as these could be performed to help find new possibilities. The techniques used in this paper are still applicable to future screens. The point remains that bacteria infect normal and antibiotic resistant cells with equal efficiency. This is a young area of research with great potential. Future experiments could start out with a broader spectrum of bacteriophages in the hopes of finding one that is more effective. They could also try passaging the viruses on multiple types of bacterial cells, and instead of looking for a broad acting bacteriophage, they could grow bacteriophages specifically for different bacterial infections. As our options of antibiotics grow slimmer and bacteria grow stronger, bacteriophage therapies could hold the answer.
Kęsik-Szeloch, Agata, Zuzanna Drulis-Kawa, Beata Weber-Dąbrowska, Jerzy Kassner, Grażyna Majkowska-Skrobek, Daria Augustyniak, Marzanna Łusiak-Szelachowska, Maciej Żaczek, Andrzej Górski, and Andrew M Kropinski. “Characterising the biology of novel lytic bacteriophages infecting multidrug resistant Klebsiella pneumoniae.” Virology. 28 March 2013; 10:100. http://www.virologyj.com/content/10/1/100/abstract.
Elander, R. P. “Industrial production of beta-lactam antibiotics.” Applied microbiology and biotechnology. 2003; 61(5–6):385–392.
Kim, Nick. Cartoon. www.nearingzero.net.
Podschun, R. and U. Ullmann. “Klebsiella spp. as Nosocomial Pathogens: Epidemiology, Taxonomy, Typing Methods, and Pathogenicity Factors.” Clin Microbiol Rev. October 1998; 11(4):589–603. http://www.ncbi.nlm.nih.gov/pmc/articles/PMC88898/.
Weinbauer, Markus G. “Ecology of prokaryotic viruses.” Microbiology Reviews. May 2004; 28(2):127–181. http://onlinelibrary.wiley.com/store/10.1016/j.femsre.2003.08.001/asset/j.femsre.2003.08.001.pdf?v=1&t=hf2z7vwz&s=532866ab4231130df544179cd2db11783463e7d6