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Updated: Sep 9, 2023


This short review is designed for medical students taking boards exams. An attempt was made to be thorough, without including excessive amounts of details. For exact antibiotic coverage, please refer to textbooks of infectious diseases or other reliable sources.


The penicillin antibiotics can be divided into four groups based on their antimicrobial activity:

  • Anti-staphylococcal penicillins (e.g., flucloxacillin, dicloxacillin, nafcillin, oxacillin)

  • Natural penicillins (e.g., penicillin G, penicillin V)

  • Aminopenicillins (e.g., ampicillin, amoxicillin)

  • Anti-pseudomonal penicillins (e.g., carbenicillin, ticarcillin, piperacillin)

Natural Penicillins

The natural penicillins (e.g., penicillin G, penicillin V) are narrow-spectrum antibiotics. Despite their narrow spectrum of activity, they are effective against a large proportion of gram-positive organisms, gram-negative cocci, and many anaerobes.


The efficacy of natural penicillins against Streptococcus pyogenes and syphilis is well documented, and because they also cover oral anaerobes, they are also very useful for odontogenic infections. Penicillin G and its variants, such as penicillin V, remain the agents of choice for a variety of infections. To date, no resistant strains of group A and B streptococci have emerged. Below is a table of some common indications for penicillin therapy.

Common indications for penicillin therapy

Group A streptococcal infections (e.g., strep throat)

Group B streptococcal infections (e.g., neonatal sepsis) Syphilis

Odontogenic infections

Pneumococcal infections (if sensitive)

Meningococcal infections (if sensitive)

This is not a complete list of indications.


Unfortunately, many other bacteria have developed resistance to these agents, including pneumococci, enterococci, meningococci, and Bacteroides spp. Also note that this class of penicillin is not effective against gram-negative rods, and because it is a penicillinase-susceptible drug, it is not effective against most staphylococcal species either.

Anti-staphylococcal penicillins

The penicillinase-resistant penicillins have a very narrow-spectrum activity and are primarily used to treat methicillin-sensitive staphylococcal aureus (MSSA). These antibiotics also have activity against streptococcal species but less so than the natural penicillins. Thus, a natural penicillin (e.g., penicillin V) is preferable to an anti-staphylococcal penicillin (e.g., oxacillin) for treating a group A beta-hemolytic Streptococcus pyogenes throat infection.

Skin Infections

Anti-staphylococcal penicillins can be used empirically for certain skin infections (e.g., impetigo) as they are active against both MSSA and streptococci. Penicillin V, however, would be a poor choice for the empiric treatment of such skin infections since they do not have staphylococcal coverage.

Severe MSSA Infections

The anti-staphylococcal penicillins are the most effective antibiotics for treating MSSA. They provide even better coverage against MSSA than antibiotics that treat MRSA (e.g., vancomycin). So for severe MSSA infections, anti-staphylococcal penicillins are a better choice than vancomycin.


These penicillins are ineffective against enterococci, anaerobes, gram-negative organisms, and methicillin-resistant staphylococcus aureus (MRSA).


The aminopenicillins have a broader spectrum of activity than the natural penicillins. This group of antibiotics retains the gram-positive coverage of penicillin G, while adding greater activity against gram-negative organisms.


The enhanced antibacterial activity of aminopenicillins allows for the treatment of a variety of different infections. For example, amoxicillin can be effective for middle ear infections caused by non-beta-lactamase-producing strains of Haemophilus influenza. It can also treat urinary tract and enteric infections if the pathogens detected are sensitive to the antibiotic. In combination with inhibitors of beta-lactamase (e.g., clavulanic acid), even wider coverage is obtained, including against MSSA, Moraxella catarrhalis, anaerobes (including Bacteroides fragilis), and gram-negative organisms (e.g., Pasteurella multocida).


Even with the addition of a beta-lactamase inhibitor, the aminopenicillins are not active against MRSA, penicillin- resistant S. pneumoniae or vancomycin-resistant enterococci.

Anti-pseudomonal penicillins

The anti-pseudomonal penicillins have extended coverage against gram-negative rods and can be used to treat infections due to Pseudomonas aeruginosa and Enterobacter species. They are also active against anaerobes, including Bacteroides fragilis, and thus can be used to treat intraabdominal infections. These antipseudomonal penicillins are also used in combination with a beta-lactamase inhibitor.


Video Lecture

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Updated: Jun 29, 2022


Impetigo is characterized by erythematous lesions that are covered with a honey-coloured crust. The lesions are usually asymptomatic, but they can be pruritic. The lesions begin as macules and papules, but then quickly fill with fluid and rupture. The lesions are highly contagious and can be transmitted to other areas of the body (i.e., autoinoculation) and to other persons. There is often a history of close contact with similar lesions.

Non-bullous impetigo

There are small vesicles that quickly burst and often are not present upon evaluation.

Bullous impetigo

There are large flaccid bullae. In both of these subtypes, the fluid filled lesions rupture and result in a superficial erosion.


The lesions extend into the dermis resulting in ulceration with violaceous borders.


A clinical diagnosis is usually made based on the characteristic skin findings. Although a gram stain and culture of the fluid within the lesion is generally recommended, treatment in typical cases can commence without these investigations. Common indications for cultures are listed in the table below.

Indications for bacterial culture and sensitivity of pus

​Local outbreak

Suspicion of MRSA

Treatment failure

​Development of poststreptoccal glomerulonephritis

Indications for nasal culture

Recalcitrant impetigo


Topical antibiotics (e.g., mupirocin, retapamulin, fuscidic cid) are recommended for infections with a limited number of lesions. Although the condition is self-limited, treatment is recommended to prevent the progression of the illness and to prevent the spread of infection to other people. Children are allowed to return to school following 24 hours of antibiotic therapy.

Indications for systemic antibiotics (e.g., cephalexin)


Widespread infection

​Systemic involvement

Local outbreak of poststreptococcal glomerulonephritis


Poststreptococcal glomerulonephritis is a potential complication of impetigo. It occurs when the pathogenic organism is a nephritogenic strain of group A beta-hemolytic streptococci.

Video Lectures


  1. Dajani AS, Ferrieri P, Wannamaker LW. Natural history of impetigo. II. Etiologic agents and bacterial interactions. J Clin Invest. 1972 Nov;51(11):2863-71. doi: 10.1172/JCI107109. PMID: 4263498; PMCID: PMC292435.

  2. Cole C, Gazewood J. Diagnosis and treatment of impetigo. Am Fam Physician. 2007 Mar 15. 75(6):859-64.

  3. Darmstadt GL, Lane AT. Impetigo: an overview. Pediatr Dermatol. 1994 Dec;11(4):293-303. doi: 10.1111/j.1525-1470.1994.tb00092.x. PMID: 7899177.

  4. Moran GJ, Amii RN, Abrahamian FM, Talan DA. Methicillin-resistant Staphylococcus aureus in community-acquired skin infections. Emerg Infect Dis. 2005 Jun. 11(6):928-30.

  5. Stevens DL, Bisno AL, Chambers HF, Dellinger EP, Goldstein EJ, Gorbach SL, Hirschmann JV, Kaplan SL, Montoya JG, Wade JC. Practice guidelines for the diagnosis and management of skin and soft tissue infections: 2014 update by the infectious diseases society of America. Clin Infect Dis. 2014 Jul 15;59(2):147-59. doi: 10.1093/cid/ciu296. Epub 2014 Jun 18. PMID: 24947530.

  6. Koning S, van der Sande R, Verhagen AP, van Suijlekom-Smit LW, Morris AD, Butler CC, Berger M, van der Wouden JC. Interventions for impetigo. Cochrane Database Syst Rev. 2012 Jan 18;1(1):CD003261. doi: 10.1002/14651858.CD003261.pub3. PMID: 22258953; PMCID: PMC7025440.

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