Bacterial biology (Forensic Science)
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...
(The entire section is 279 words.)
Forensic Applications (Forensic Science)
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...
(The entire section is 384 words.)
Further Reading (Forensic Science)
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...
(The entire section is 188 words.)
Bacterial Biology (World of Forensic Science)
The Dutch merchant and amateur scientist Anton van Leeuwenhoek was the first to observe bacteria and other microorganisms. Using single-lens microscopes of his own design, he described bacteria and other microorganisms as "animacules."
An understanding of the fundamentals of bacterial biology is critical to bacteriologists and other forensic investigators attempting to identify potential biogenic pathogens that may be exploited by bioterrorists. In addition, the reaction of the body to bacteria and the type of bacteria present often offer invaluable clues to forensic investigators.
Bacteria are one-celled prokaryotic organisms that lack a true nucleus (i.e., a nucleus defined by a membrane). Bacteria maintain their genetic material, deoxyribonucleic acid (DNA), in a single, circular chain. Bacteria also contain DNA in small circular molecules termed plasmids.
In addition to not being contained in a membrane bound nucleus, the DNA of prokaryotes is not associated with the special chromosome proteins called histones, which are found in higher organisms. In addition, prokaryotic cells lack other membrane-bounded organelles, such as mitochondria.
Although all bacteria share certain structural, genetic, and metabolic characteristics, important biochemical differences exist among the many species of bacteria. The cytoplasm of all bacteria is enclosed within a cell membrane surrounded by a rigid cell wall whose polymers, with few exceptions, include peptidoglycansarge, structural molecules made of protein carbohydrate. Bacteria also secrete a viscous, gelatinous polymer (called the glycocalyx) on their cell surfaces. This polymer, composed either of polysaccharide, polypeptide, or both, is called a capsule when it occurs as an organized layer firmly attached to the cell wall. Capsules increase the disease-causing ability (virulence) of bacteria by inhibiting immune system cells called phagocytes from engulfing them.
During the 1860s, the French microbiologist Louis Pasteur studied fermenting bacteria. He demonstrated that fermenting bacteria could contaminate wine and beer during manufacturing, turning the alcohol produced by yeast into acetic acid (vinegar). Pasteur also showed that heating the beer and wine to kill the bacteria preserved the flavor of these beverages. The process of heating, now called pasteurization in his honor, is still used to kill bacteria in some alcoholic beverages, as well as milk.
Pasteur described the spoilage by bacteria of alcohol during fermentation as being a "disease" of wine and beer. His work was thus vital to the later idea that human diseases could also be caused by microorganisms, and that heating can destroy them.
The first antibiotic (a substance designed to kill bacteria) was penicillin, discovered in 1928 by Sir Alexander Fleming. Since then, a myriad of naturally occurring and chemically synthesized antibiotics have been used to control bacteria. Introduction of an antibiotic is frequently followed by the development of resistance to the agent. Resistance is an example of the adaptation of the bacteria to the anti-bacterial agent.
Bacteria can multiply and cause an infection in the bloodstream. The invasion of the bloodstream by the particular type of bacteria is referred to as a bacteremia. If the invading bacteria also release toxins into the bloodstream, the malady can also be called blood poisoning or septicemia. Staphylococcus and Streptococcus are typically associated with septicemia.
The bloodstream is susceptible to invasion by bacteria that may gain entry in several ways, including: via a wound or abrasion in the protective skin overlay of the body; as a result of another infection elsewhere in the body; following the introduction of bacteria during a surgical procedure; or via a needle during injection of a drug.
Depending on the identity of the infecting bacterium and on the physical state of the human host (primarily with respect to the efficiency of the immune system), bacteremic infections may not produce any symptoms. However, some infections do produce symptoms, ranging from an elevated temperature, as the immune system copes with the infection, to a spread of the infection to the heart (endocarditis or pericarditis) or the covering of nerve cells (meningitis). In more rare instances, a bacteremic infection can produce a condition known as septic shock. The latter occurs when the infection overwhelms the ability of the body's defense mechanisms to cope. Septic shock can be lethal.
Septicemic infections usually result from the spread of an established infection. Bacteremic (and septicemic) infections often arise from bacteria that are normal residents on the surface of the skin or internal surfaces, such as the intestinal tract epithelial cells. In their normal environments the bacteria are harmless and even can be beneficial. However, if they gain entry to other parts of the body, these socalled commensal bacteria can pose a health threat. The entry of these commensal bacteria into the bloodstream is a normal occurrence for most people. In the majority of people, however, the immune system is more than able to deal with the invaders. Yet if the immune system is not functioning efficiently, the invading bacteria may be able to multiply and establish an infection. Examples of conditions that compromise the immune system are another illness (such as acquired immunodeficiency syndrome and certain types of cancer), certain medical treatments such as irradiation, and the abuse of drugs or alcohol.
Examples of bacteria that are most commonly associated with bacteremic infections are Staphylococcus, Streptococcus, Pseudomonas, Haemophilus, and Escherichia coli.
The generalized location of bacteremia produces generalized symptoms. These symptoms can include a fever, chills, pain in the abdomen, nausea with vomiting, and a general feeling of ill health. Not all these symptoms are necessarily present at the same time. The nonspecific nature of the symptoms may not prompt a physician to suspect bacteremia until the infection is more firmly established. Septic shock produces more drastic symptoms, including elevated rates of breathing and heartbeat, loss of consciousness, and failure of organs throughout the body. The onset of septic shock can be rapid, so prompt medical attention is critical.
As with many other infections, bacteremic infections can be prevented by observance of proper hygienic procedures including hand washing, cleaning of wounds, and cleaning sites of injections to temporarily free the surface of living bacteria. The rate of bacteremic infections due to surgery is much less now than in the past, due to the advent of sterile surgical procedures, but is still a serious concern.
Bacterial infection does not always result in diseaseven if the pathogen is virulent (able to cause disease). The steps of pathogenesis (the process of causing actual disease), can depend on a number of genetic and environmental factors. In some cases, pathogenic bacteria produce toxins released extracellularly (exotoxins) that migrate from the actual site of infection to cause damage to cells in other parts of the body.
Evidence of bacteremic infections can provide forensic investigators with valuable clues about the nature of wounds, the time wounds were inflicted, and even specifics about wound care after injury.
SEE ALSO Anthrax; Bacteria, classification; Bacteria, growth and reproduction; Bacterial resistance and response to antibacterial agents; Biological weapons, genetic identification; Biosensor technologies; Bubonic plague; Decontamination methods.