Bacterial biology
Bacterial biology is the study of prokaryotic organisms, which are simple, single-celled entities lacking membrane-bound organelles and nuclei. These microscopic organisms reproduce by cell division, resulting in identical daughter cells. Bacteria play a significant role in various fields, including forensic science, where their identification is crucial in cases of hospital-acquired infections, food-borne illnesses, and even bioterrorism. Researchers utilize DNA polymorphisms, such as single nucleotide polymorphisms (SNPs), variable number of tandem repeats (VNTRs), and short tandem repeats (STRs), as markers for differentiating bacterial strains.
Techniques like Polymerase Chain Reaction (PCR) are employed to amplify specific DNA sequences, which aids in identifying bacteria responsible for infections. Additionally, the analysis of ribosomal RNA (rRNA) sequences helps in classifying different types of bacteria, contributing to our understanding of microbial diseases. The identification of antibiotic-resistant strains, such as methicillin-resistant Staphylococcus aureus, is vital for effective treatment. Furthermore, bacterial profiling is essential for tracing the sources of food-borne infections and is also applied in investigations related to biological threats. The study of bacterial growth patterns post-mortem can even provide insights into estimating time of death. Overall, bacterial biology is a crucial area of research with significant implications for public health, safety, and forensic investigations.
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Bacterial biology
Definition: Study of prokaryotic organisms that lack membrane-bound organelles and nuclei—simple, single-celled microscopic organisms that grow by cell division to produce identical daughter cells.
Significance: Forensic scientists are sometimes called upon to identify the bacterial strains that caused such problems as hospital-acquired infections, food-borne infections, or microbial diseases; they may also need to identify the biological agents used in acts of bioterrorism. Bacteria have different DNA polymorphisms (variations in DNA sequence between individual bacteria or bacterial strains) that serve as markers for typing different bacteria.
Several types of polymorphisms are used for DNA (deoxyribonucleic acid) profiling. One type is single nucleotide polymorphisms (SNPs), in which only a single nucleotide in a sequence varies. A second type is variable number of tandem repeats (VNTRs). A sequence of DNA is tandemly (end-to-end) repeated, with the number of repeats differing between individual bacteria. An example is a sequence of thirty nucleotides that is repeated between twenty to one hundred times in different bacterial cells. To identify VNTRs in bacteria, Polymerase chain reaction (PCR) primers are designed for both sides of the VNTR locus. With PCR, the sequence between the two primers is amplified, giving a large amount of this specific DNA, which is then separated by gel electrophoresis to determine the size (number of repeats) of the region amplified. The different numbers of tandem repeats are thought to arise from mistakes in DNA replication that generate INDEL (insertion or deletion of DNA) mutations.
An additional polymorphism is short tandem repeats (STRs), which are short sequence elements that repeat themselves within the DNA molecule. The repeating sequence is usually three to seven bases in length, and the entire length of an STR is fewer than five hundred bases in length.
Other types of markers used to identify bacteria are the sequences of 16S rRNA (ribosomal ribonucleic acid) and the spacer between the 16 and 23S rRNAs. Ribosomal RNA is part of the ribosome that translates messenger RNA into proteins. By comparing rRNA sequences, scientists can identify types of bacteria. PCR is used to amplify the specific DNA coding for the 16S rRNA. For example, 16S rRNA can be used to identify the bacterial pathogen causing disease in different persons.
Forensic Applications
The ability to identify bacteria is important in many kinds of cases. For example, when patients develop infections while in the hospital, this can pose a particular problem because of the extensive use of antibiotics and the development of antibiotic-resistant bacterial strains such as methicillin-resistant Staphylococcus aureus, which is seen in hospital-acquired infections. The different strains of Staphylococcus can be identified through DNA typing. The identification of an antibiotic-resistant strain of a bacterium leads to a more effective type of antibiotic treatment for the patient. Also, in some cases infections may be caused by inadequate hygienic precautions taken during surgery or in postoperative care. DNA analysis is important to identify the source of such infection-causing bacterial strains.
In cases of food-borne infections, it is important to be able to trace the microbes that caused them to the sources—whether companies, farms, or persons—to determine the origin of the microbes. Scientists use DNA analysis to track food-borne infections caused by Salmonella or the Esherichia coli strain O157:H7 to identify the types of bacteria causing the problems.
Molecular techniques are used to follow outbreaks of microbial diseases. The U.S. Centers for Disease Control and Prevention (CDC) maintains a database of microbial DNA fingerprints (PulseNet). Scientists have examined some thirty-one VNTR loci to compare strains of Mycobacterium tuberculosis, the bacterium that causes tuberculosis.
It is also important to be able to identify bacteria in cases of biological terrorism. For example, in 2001, letters containing Bacillus anthracis, the bacterium that causes anthrax, were sent through the mail in the eastern United States, and five people died of inhalation anthrax. Because B. anthracis spores are commonly found in soil, it was essential that prosecutors prove that spores found in a suspect’s home or laboratory were the same strain that was found on the material mailed to the victims. In 2002, the American Academy of Microbiology met to formulate standards for evidence collection and analysis of molecular tests for microbial forensics.
Bacteria can also be used to estimate time of death. After death, the action of bacteria destroys the soft tissues of the body. The bacteria generally found are those normally present in the respiratory and intestinal tracts, such as bacilli, coliform, and clostridiuim. The temperature of the environment surrounding the body determines the rate of bacterial growth.
Bibliography
Breeze, Roger G., Bruce Budowle, and Steven E. Schutzer, eds. Microbial Forensics. Burlington, Mass.: Elsevier Academic Press, 2005. Details the importance of forensic microbiology and discusses its uses.
Butler, John M. Forensic DNA Typing: Biology, Technology, and Genetics of STR Markers. 2d ed. Burlington, Mass.: Elsevier Academic Press, 2005. Accessible textbook provides a detailed overview of DNA methodologies used by forensic scientists.
Cho, Mildred K., and Pamela Sankar. “Forensic Genetics and Ethical, Legal, and Social Implications Beyond the Clinic.” Nature Genetics 36 (2004): S8-S12. Discusses the ethical considerations related to DNA profiling and genetic analysis.
Jobling, Mark A., and Peter Gill. “Encoded Evidence: DNA in Forensic Analysis.” Nature Reviews Genetics 5 (October, 2004): 739-751. Provides an informative summary of DNA forensics.
Kobilinsky, Lawrence F., Thomas F. Liotti, and Jamel Oeser-Sweat. DNA: Forensic and Legal Applications. Hoboken, N.J.: Wiley-Interscience, 2005. Presents a general overview of the uses of DNA analysis and profiling.
Madigan, Michael T., John M. Martinko, Paul V. Dunlap, and David P. Clark. Brock Biology of Microorganisms. 12th ed. Upper Saddle River, N.J.: Pearson Prentice Hall, 2008. Widely respected basic microbiology textbook includes information about biological weapons and methods of microbial identification.