Porth's Essentials of Pathophysiology, 4e

314

Infection and Immunity

U N I T 4

DENATURATION

ANNEALING

EXTENSION

PRODUCTS

Enzyme, dNTPs, dye-labeled terminators

A A A A A A A

C C C C C C

C C C C C

C A G T

G G G G

T

A T T A

T

FIGURE 14-12. Sanger sequencing. Sanger sequencing uses PCR to amplify DNA fragments and labeled nucleotides as DNA elongation terminators.The DNA sample is divided into four separate sequencing reactions; to each reaction one of the four dideoxynucleotide terminators (ddATP, ddGTP, ddCTP, or ddTTP) are added to terminate DNA strand extension at random length, resulting in DNA fragments of varying lengths.The newly labeled DNA fragments are denatured, and separated by size (with a resolution of just one nucleotide) by gel or capillary electrophoresis. dNTPs, deoxynucleoside triphosphates. (Copyright © 2014 LifeTechnologies Corporation; www.lifetechnologies.com. Used under permission.)

Treatment of Infectious Diseases

than 10 years to complete at a cost of more than $3 billon. Today newer sequencing methods, collectively known as Next Generation Sequencing (NGS), have allowed laboratories to sequence whole genomes of bacteria, molds, mycobacteria, and even humans at a fraction of the cost and within days rather than decades. For infectious diseases, the applications of sequencing are exciting and impactful. In epidemiol- ogy, next-generation sequencing will likely replace our current laboratory methods. For example, in 2011 a new strain of E. coli was detected in an out- break of serious foodborne illness beginning in north- ern Germany that ultimately affected nearly 4000 patients with 53 dead. Within 1 week of isolating the new strain of E. coli , clinical microbiologists had grown the organism, sequenced it, and identified the source of the outbreak. With powerful laboratory technologies like sequencing, epidemiologists will be better equipped to identify outbreaks early or even prevent them from occurring.

The goal of treatment for an infectious disease is com- plete removal of the pathogen from the host and the restoration of normal physiologic function to damaged tissues. Most infectious diseases of humans are self-lim- iting in that they require little or no medical therapy for a complete cure. When an infectious process gains the upper hand and therapeutic intervention is essential, the choice of treatment may be medicinal through the use of antimicrobial agents or surgical by removing infected tis- sues. The decision about which therapeutic modality or combination of therapies to use is based on the extent, urgency, and location of the disease process; the patho- gen; and the availability of effective antimicrobial agents. Antibacterial Agents Antibacterial agents are generally called antibiotics. Most antibiotics are actually produced by other micro- organisms, primarily bacteria and fungi, as by-products of metabolism, and usually are effective only against other prokaryotic organisms. An antibiotic is consid- ered bactericidal if it causes irreversible and lethal dam- age to the bacterial pathogen and bacteriostatic if its inhibitory effects on bacterial growth are reversed when the agent is eliminated. Antibiotics can be classified into families of compounds with related chemical structure and activity. Not all antibiotics are effective against all patho- genic bacteria. Some agents are effective only against gram-negative bacteria, and others only gram-positive bacteria. The so-called broad-spectrum antibiotics , such as the newer cephalosporins, are active against a wide variety of gram-positive and gram-negative bac- teria. The four basic mechanisms of antibiotic action

SUMMARY CONCEPTS

■■ The diagnosis of infectious disease relies on the recovery of the probable pathogen or evidence of its presence from infected sites in the host and accurate documentation of signs and symptoms compatible with an infectious process. ■■ In the laboratory, the diagnosis of an infectious agent is accomplished through the use of culture, serology, and DNA/RNA sequencing techniques.