Antimicrobial agents can be categorized as antibacterial, antifungal, antiparasitic, or antiviral agents because each group is used to treat infections caused by bacteria, fungus, parasites or viruses. Further classifications within those categories are used to differentiate between agents effective against specific pathogens. For example, aminoglycosides are primarily used to treat gram-negative infections while antianaerobic agents are used to cure infections caused by gram-positive bacteria and anaerobic infections (Arcangelo & Peterson, 2013).
Viral and bacterial infections are different, but the differences between them are not always obvious. As a general rule, bacterial infections usually manifest as strep throat, urinary tract infections, and tuberculosis while viral infections include the common cold, chickenpox, and AIDS. However, both types of microbes can be responsible for the same conditions, such as skin infections and pneumonia.
In pneumonia, it is difficult to determine the exact cause of the infection with conventional laboratory tests, and molecular tests, such as the polymerase chain reaction are not implemented in all clinical routines (Parameswaran & Sethi, 2012). Although various other factors, such as radiographic changes, symptoms, and response to therapy, can help the physicians differentiate between viral and bacterial infections, choosing a suitable antimicrobial agent is difficult without a specific clinical algorithm (Ruuskanen, Lahti, Jennings, & Murdoch, 2011).
The infection acquisition and the pathophysiology of the infection are often the key differences between viral and bacterial infections. Bacteria are single-cell organisms while viruses require host cells to proliferate, so viruses are spread exclusively through interpersonal contact. Bacteria are usually a regular part of the flora in the human body, but they become hazardous once they spread into locations where they do not belong and proliferate, so bacterial infections can be both community-acquired and opportunistic disorders (Arcangelo & Peterson, 2013).
Several factors can determine the choice of antimicrobial agents for treatment, including efficacy, toxicity, and cost, but the most important factor is the patient’s medical history and the source of the infection (Arcangelo & Peterson, 2013). For example, a history of diabetes or immunosuppression disorders may determine a possible predisposition for acquiring a certain pathogen (Arcangelo & Peterson, 2013). The location in which the patient acquired the disorder, such as hospital setting or community, can also help reduce the list of responsible pathogens. Drug therapy is usually initiated before the microbiology laboratory results determine the type of pathogen involved in the infection and susceptibility testing (Arcangelo & Peterson, 2013). That approach is common in critically ill and hospitalized patients because obtaining the results can take up to 72 hours, so the empiric antimicrobial therapy covers the broad range of most likely pathogens to reduce mortality and morbidity rates (Leekha, Terrell, & Edson, 2011).
Empiric antimicrobial therapy, which usually includes multiple antimicrobial agents, is also used to prevent antimicrobial resistance development or treat polymicrobial infections. An example of empiric antimicrobial therapy in bacterial meningitis would include cephalosporin and vancomycin because Streptococcus pneumoniae and Neisseria meningitidis are the most common pathogens in that infection (Leekha et al., 2011). Definitive therapy is initiated once the exact pathogen is established.
However, differentiating between bacterial and viral infections is critical because using the wrong antimicrobial agents can produce adverse events. For example, antibiotics are ineffective against viruses, and wrong use can lead to the development of bacterial strains that are resistant to the type of antibiotic used (Leekha et al., 2011). Several new methods, such as procalcitonin level measurement and the nucleic acid tests, can now help healthcare providers differentiate better between viral and bacterial infections, but those tests still need to be integrated as standard practices in healthcare (Leekha et al., 2011).
Because of the physiological differences between bacteria and viruses, antimicrobial agents need different mechanisms of action to effectively address the causes of infection. While the role of antibiotics is to kill or reduce the amount of bacteria causing the infection, antiviral agents aim at containing the viruses in infected cells or preventing their proliferation. For example, neuroaminidase inhibitors are antiviral agents that inhibit the neuraminidase protein, so they prevent the release of influenza A and B viruses from infected cells (Arcangelo & Peterson, 2013). Integrase strand transfer inhibitors, such as Isentress, block the integrase enzyme in the human immunodeficiency virus, so it cannot use host cells to replicate (Cerner Multum, 2012).
References
Arcangelo, V. P., & Peterson, A. M. (Eds.). (2013). Pharmacotherapeutics for advanced practice: A practical approach (3rd ed.). Ambler, PA: Lippincott Williams & Wilkins.
Cerner Multum. (2012). Isentress. Retrieved from http://www.drugs.com/isentress.html
Leekha, S., Terrell, C. L., & Edson, R. S. (2011). General principles of antimicrobial therapy. Mayo Clinic Proceedings, 86(2), 156-167. doi:10.4065/mcp.2010.0639
Parameswaran, G. I. & Sethi, S. (2012). Viral pneumonia. In: S. Spiro, G. Silvestri, & A. Agusti (Eds.), Clinical respiratory medicine (4th ed.) (pp. 309-314). Philadelphia, PA: W.B. Saunders.
Ruuskanen, O., Lahti, E., Jennings, L. C., & Murdoch, D. R. (2011). Viral pneumonia. The Lancet, 377(9773), 1264-1275.
