Drugs that weaken the bacterial cell wall I: penicillins

Introduction to the penicillins

The penicillins are practically ideal antibiotics. Why? Because they are active against a variety of bacteria and their direct toxicity is low. Allergic reactions are the principal adverse effects. Owing to their safety and efficacy, the penicillins are widely prescribed.

Because they have a beta-lactam ring in their structure (Fig. 84–1), the penicillins are known as beta-lactam antibiotics. The beta-lactam family also includes the cephalosporins, carbapenems, and aztreonam (see Chapter 85). All of the beta-lactam antibiotics share the same mechanism of action: disruption of the bacterial cell wall.

Figure 84–1 Structural formulas of representative penicillins.
The unique structure of individual penicillins is determined by the side chain coupled to the penicillin nucleus at the position labeled R. This side chain influences acid stability, pharmacokinetic properties, penicillinase resistance, and ability to bind specific penicillin-binding proteins.

Mechanism of action

To understand the actions of the penicillins, we must first understand the structure and function of the bacterial cell wall—a rigid, permeable, mesh-like structure that lies outside the cytoplasmic membrane. Inside the cytoplasmic membrane, osmotic pressure is very high. Hence, were it not for the rigid cell wall, which prevents expansion, bacteria would take up water, swell, and then burst.

Penicillins weaken the cell wall, causing bacteria to take up excessive amounts of water and rupture. As a result, penicillins are generally bactericidal. However, it is important to note that penicillins are active only against bacteria that are undergoing growth and division (see below).

Penicillins weaken the cell wall by two actions: (1) inhibition of transpeptidases and (2) disinhibition (activation) of autolysins. Transpeptidases are enzymes critical to cell wall synthesis. Specifically, they catalyze the formation of cross-bridges between the peptidoglycan polymer strands that form the cell wall, and thereby give the cell wall its strength (Fig. 84–2). Autolysins are bacterial enzymes that cleave bonds in the cell wall. Bacteria employ these enzymes to break down segments of the cell wall to permit growth and division. By simultaneously inhibiting transpeptidases and activating autolysins, the penicillins (1) disrupt synthesis of the cell wall and (2) promote its active destruction. These combined actions result in cell lysis and death.

Figure 84–2 Inhibition of transpeptidase by penicillins.
The bacterial cell wall is composed of long strands of a peptidoglycan polymer. As depicted, transpeptidase enzymes create cross-bridges between the peptidoglycan strands, giving the cell wall added strength. By inhibiting transpeptidases, penicillins prevent cross-bridge synthesis and thereby weaken the cell wall.

The molecular targets of the penicillins (transpeptidases, autolysins, other bacterial enzymes) are known collectively as penicillin-binding proteins (PBPs). These molecules are so named because penicillins must bind to them to produce antibacterial effects. As indicated in Figure 84–3, PBPs are located on the outer surface of the cytoplasmic membrane. More than eight different PBPs have been identified. Of these, PBP1 and PBP3 are most critical to penicillin’s antibacterial effects. Bacteria express PBPs only during growth and division. Accordingly, since PBPs must be present for penicillins to work, these drugs work only when bacteria are growing.

Figure 84–3 The bacterial cell envelope.
Note that the gram-negative cell envelope has an outer membrane, whereas the gram-positive envelope does not. The outer membrane of the gram-negative cell envelope prevents certain penicillins from reaching their target molecules. (PBP = penicillin-binding protein [transpeptidases and other penicillin target molecules], • = beta-lactamases.)

Since mammalian cells lack a cell wall, and since penicillins act specifically on enzymes that affect cell wall integrity, the penicillins have virtually no direct effects on cells of the host. As a result, the penicillins are among our safest antibiotics.

Mechanisms of bacterial resistance

Bacterial resistance to penicillins is determined primarily by three factors: (1) inability of penicillins to reach their targets (PBPs), (2) inactivation of penicillins by bacterial enzymes, and (3) production of PBPs that have a low affinity for penicillins.

The gram-negative cell envelope

All bacteria are surrounded by a cell envelope. However, the cell envelope of gram-negative organisms differs from that of gram-positive organisms. Because of this difference, some penicillins are ineffective against gram-negative bacteria.

As indicated in Figure 84–3, the cell envelope of gram-positive bacteria has only two layers: the cytoplasmic membrane plus a relatively thick cell wall. Despite its thickness, the cell wall can be readily penetrated by penicillins, giving them easy access to PBPs on the cytoplasmic membrane. As a result, penicillins are generally very active against gram-positive organisms.

The gram-negative cell envelope has three layers: the cytoplasmic membrane, a relatively thin cell wall, and an additional outer membrane (see Fig. 84–3). Like the gram-positive cell wall, the gram-negative cell wall can be easily penetrated by penicillins. The outer membrane, however, is difficult to penetrate. As a result, only certain penicillins (eg, ampicillin) are able to cross it and thereby reach PBPs on the cytoplasmic membrane.

Penicillinases (beta-lactamases)

Beta-lactamases are enzymes that cleave the beta-lactam ring, and thereby render penicillins and other beta-lactam antibiotics inactive (Fig. 84–4). Bacteria produce a large variety of beta-lactamases; some are specific for penicillins, some are specific for other beta-lactam antibiotics (eg, cephalosporins), and some act on several kinds of beta-lactam antibiotics. Beta-lactamases that act selectively on penicillins are known as penicillinases.

Figure 84–4 The effect of beta-lactamase on the penicillin nucleus.

Penicillinases are synthesized by gram-positive and gram-negative bacteria. Gram-positive organisms produce large amounts of these enzymes, and then export them into the surrounding medium. In contrast, gram-negative bacteria produce penicillinases in relatively small amounts, and, rather than exporting them to the environment, secrete them into the periplasmic space (see Fig. 84–3).

The genes that code for beta-lactamases are located on chromosomes and on plasmids (extrachromosomal DNA). The genes on plasmids may be transferred from one bacterium to another, thereby promoting the spread of penicillin resistance.

Transfer of resistance is of special importance with Staphylococcus aureus. When penicillin was first introduced in the early 1940s, all strains of Staph. aureus were sensitive. However, by 1960, as many as 80% of Staph. aureus isolates in hospitals displayed penicillin resistance. Fortunately, a penicillin derivative (methicillin) that has resistance to the actions of beta-lactamases was introduced at this time. To date, no known strains of Staph. aureus produce beta-lactamases capable of inactivating methicillin or related penicillinase-resistant penicillins (although some strains are resistant to these drugs for other reasons).

Altered penicillin-binding proteins

Certain bacterial strains, known collectively as methicillin-resistant Staphylococcus aureus (MRSA), have a unique mechanism of resistance: production of PBPs with a low affinity for penicillins and all other beta-lactam antibiotics. How did MRSA develop this ability? By acquiring genes that code for low-affinity PBPs from other bacteria. Infection with MRSA and its management are discussed in Box 84–1.

BOX 84–1 SPECIAL INTEREST TOPIC

METHICILLIN-RESISTANT STAPHYLOCOCCUS AUREUS

Staphylococcus aureus is a gram-positive bacterium that often colonizes the skin and nostrils of healthy people. Infection usually involves the skin and soft tissues, causing abscesses, boils, cellulitis, and impetigo. However, more serious infections can also develop, including infections of the lungs and bloodstream, which can be fatal.

Like other pathogens, Staph. aureus has developed resistance over the years. When penicillins were introduced in the 1940s, all strains of Staph. aureus were susceptible. However, penicillin-resistant strains quickly emerged, owing to bacterial production of penicillinases. In 1959, this resistance was overcome with methicillin, the first penicillinase-resistant penicillin. Unfortunately, by 1968, strains resistant to methicillin had emerged. These highly resistant bacteria, known as methicillin-resistant Staph. aureus (MRSA), are resistant not only to methicillin (now obsolete), but to all penicillins and all cephalosporins as well. The basis of MRSA resistance is acquisition of genes that code for penicillin-binding proteins that have very low affinity for penicillins and cephalosporins. Resistant strains were initially limited to healthcare facilities, but are now found in the community as well.

In the United States, MRSA is a serious public health problem. Between 2000 and 2005, hospital stays for MRSA infection tripled, rising from 128,500 to 368,800. In 2005, MRSA caused an estimated 94,360 severe (ie, invasive) infections, resulting in 18,650 deaths—6150 more than were caused by HIV/AIDS. Not only does MRSA increase mortality, it increases costs: Treating hospitalized MRSA patients runs about $35,000, compared with $14,000 for patients with methicillin-sensitive infections. Fortunately, the MRSA news isn’t all bad. For one thing, although MRSA infections are now common, most patients can be cured. Also, rates of MRSA infection among hospitalized patients are now falling, after rising steadily for many years.

There are two distinct types of MRSA, referred to as hospital-associated MRSA (HA-MRSA) and community-associated MRSA (CA-MRSA). Of the two, HA-MRSA is more prevalent (85% vs. 15%) and emerged earlier (1968 vs. 1981). Also, HA-MRSA infection is generally more serious and harder to treat. Molecular typing indicates that HA-MRSA and CA-MRSA are genetically distinct strains, known as USA100 and USA300, respectively.

Hospital-associated MRSA

Methicillin resistance in Staph. aureus was first reported in isolates from hospitalized patients. The year was 1968. For most of the next four decades, the prevalence of HA-MRSA among hospitalized patients climbed steadily, reaching 85% of all invasive Staph. aureus infections by 2004. However, rates of HA-MRSA are now falling: Between 2005 and 2008, the prevalence of hospital-onset invasive MRSA infections fell by an average of 9.4% a year.

Although many infections with HA-MRSA surface in the community, nearly all occur in people who had been exposed to a healthcare facility within the prior year, indicating that acquisition of the infection probably occurred in a healthcare setting—not out in the community. Transmission of HA-MRSA is usually through person-to-person contact, very often between healthcare workers and patients. Risk factors for acquiring HA-MRSA include old age, recent surgery or hospitalization, dialysis, treatment in an ICU, prolonged antibiotic therapy, an indwelling catheter, and residence in a long-term care facility.

How do we treat HA-MRSA infection? The issue is addressed at length in a new guideline—Clinical Practice Guidelines by the Infectious Diseases Society of America for the Treatment of Methicillin-Resistant Staphylococus Aureus Infections in Adults and Children—issued January 5, 2011. The guideline stresses the importance of selecting drugs based on the site of the infection, age of the patient, and drug sensitivity of the pathogen. For complicated skin and soft tissue infections in adults, the preferred drugs are IV vancomycin, linezolid [Zyvox], daptomycin [Cubicin], telavancin [Vibativ], clindamycin, and ceftaroline [Teflaro]. Intravenous vancomycin is the preferred drug for children. For bacteremia or endocarditis in adults or children, IV vancomycin and daptomycin are drugs of choice. Preferred drugs for pneumonia in adults and children are IV vancomycin, linezolid, and clindamycin. Because most strains of MRSA are multidrug resistant, many other antibiotics are ineffective, including tetracyclines, clindamycin, trimethoprim/sulfamethoxazole, and beta-lactam agents (except ceftaroline).

Community-associated MRSA

Infection with CA-MRSA, first reported in 1981, is caused by staphylococcal strains that are genetically distinct from HA-MRSA. For example, most strains of CA-MRSA carry a gene for Panton-Valentine leukocidin (a cytotoxin that causes necrosis), whereas HA-MRSA strains do not. Many people are now asymptomatic carriers of CA-MRSA. In fact, between 20% and 30% of the population is colonized, typically on the skin and in the nostrils.

Infection with CA-MRSA is generally less dangerous than with HA-MRSA, but more dangerous than with methicillin-sensitive Staph. aureus. In most cases, CA-MRSA causes mild infections of the skin and soft tissues, manifesting as boils, impetigo, and so forth. However, CA-MRSA can also cause more serious infections, including necrotizing fasciitis, severe necrotizing pneumonia, and severe sepsis. Fortunately, these invasive infections are relatively rare. On the other hand, infections of the skin and soft tissues are now common, with CA-MRSA accounting for more than 50% of the Staph. aureus isolates from these sites.

How is CA-MRSA transmitted? And who is vulnerable? Transmission is by skin-to-skin contact, and by contact with contaminated objects, including frequently touched surfaces, sports equipment, and personal items (eg, razors). In contrast to HA-MRSA infection, CA-MRSA infection is seen primarily in young, healthy people with no recent exposure to healthcare facilities. Individuals at risk include athletes in contact sports (eg, wrestling), men who have sex with men, and people who live in close quarters, such as family members, day care clients, prison inmates, military personnel, and college students.

Several measures can reduce the risk of CA-MRSA transmission. Topping the list is good hand hygiene—washing with soap and water or applying an alcohol-based sanitizer. Other measures include showering after contact sports, cleaning frequently touched surfaces, keeping infected sites covered, and not sharing towels and personal items.

Treatment depends on infection severity. For boils, small abscesses, and other superficial infections, surgical drainage may be all that is needed. For more serious infections, drugs may be indicated. Preferred agents are trimethoprim/sulfamethoxazole, minocycline, doxycycline, and clindamycin. Alternative drugs—vancomycin, daptomycin, and linezolid—should be reserved for severe infections and treatment failures. To eradicate the carrier state, intranasal application of a topical antibiotic—mupirocin or retapamulin—can be effective. Like HA-MRSA, CA-MRSA does not respond to beta-lactam antibiotics, except ceftaroline.

Chemistry

All of the penicillins are derived from a common nucleus: 6-aminopenicillanic acid. As shown in Figure 84–1, this nucleus contains a beta-lactam ring joined to a second ring. The beta-lactam ring is essential for antibacterial actions. Properties of individual penicillins are determined by additions made to the basic nucleus, primarily at the site labeled R. These modifications determine (1) affinity for PBPs, (2) resistance to penicillinases, (3) ability to penetrate the gram-negative cell envelope, (4) resistance to stomach acid, and (5) pharmacokinetic properties.

Classification

The most useful classification of penicillins is based on antimicrobial spectrum. When classified this way, the penicillins fall into four major groups: (1) narrow-spectrum penicillins that are penicillinase sensitive, (2) narrow-spectrum penicillins that are penicillinase resistant (antistaphylococcal penicillins), (3) broad-spectrum penicillins (aminopenicillins), and (4) extended-spectrum penicillins (antipseudomonal penicillins). Table 84–1 lists the members of each group and their principal target organisms.

TABLE 84–1

Classification of the Penicillins

Penicillin Class	Drug	Clinically Useful Antimicrobial Spectrum
Narrow-spectrum penicillins: penicillinase sensitive	Penicillin G Penicillin V	Streptococcus species, Neisseria species, many anaerobes, spirochetes, others
Narrow-spectrum penicillins: penicillinase resistant (antistaphylococcal penicillins)	Methicillin* Nafcillin Oxacillin Dicloxacillin	Staphylococcus aureus
Broad-spectrum penicillins (aminopenicillins)	Ampicillin Amoxicillin	Haemophilus influenzae, Escherichia coli, Proteus mirabilis, enterococci, Neisseria gonorrhoeae
Extended-spectrum penicillins (antipseudomonal penicillins)	Ticarcillin^† Piperacillin	Same as broad-spectrum penicillins plus Pseudomonas aeruginosa, Enterobacter species, Proteus (indole positive), Bacteroides fragilis, many Klebsiella