Figure 114 Effect of lactamases on susceptible cephalosporins

the p-lactam ring (see Figure 11.3). The situation with susceptible cephalosporins is more complex. Opening of the p-lactam ring again occurs (11.26) and this is accompanied by expulsion of the group at R2 (except in cephalexin, where R2 is H). The molecule finally breaks up into fragments (see Figure 11.4).

Extensive studies have shown that several types of p-lactamases exist among Gram-negative bacteria. These have been classified into several different groups on the basis of their substrate profile (the rate at which different p-lactam substrates are inactivated) and whether their activity is inhibited by p-chloromercuribenzoate (PCMB) and cloxacillin. A simplified summary of the different types is presented in Table 11.4.

With several of the newer cephalosporins, Vmax values for hydrolysis by chromosomal cephalosporinases are extremely low. However Km values demonstrate that these antibiotics have very high affinity for these enzymes. It has ben proposed that 'enzyme trapping' in the periplasm, without drug hydrolysis, could be a resistance mechanism, but this is likely only where considerable amounts of p-lactamase are produced and the organism is poorly permeable to the antibiotic and the drug itself poorly diffusible.

11.4.1.2 Aminoglycoside-modifying enzymes

P-lactam antibiotics are destroyed by P-lactamases. In contrast, aminoglycoside antibiotics are not totally inactivated by plasmid-encoded enzymes but rather modified in the outer regions of the resistant cell and are thus not bound to the target (ribosome). Only a small proportion of the external aminoglycoside need be modified for resistance to be expressed.

Aminoglycoside-modifying enzymes are of three types:

(i) acetyltransferases (AAC), which transfer an acetyl group from acetylcoenzyme A to susceptible -NH2 groups in the antibiotic;

(ii) adenylyltransferases (AAD), which transfer adenosine monophosphate (AMP) from adenosine triphosphate (ATP) to susceptible -OH groups in the antibiotic;

(iii) phosphotransferases (APH), which phosphorylate susceptible -OH groups, ATP acting as the source of phosphate.

Table 11.4 Types of beta-lactamases.

Group of Preferred Inhibited by: Representative enzyme1 súfrate Clavulanic EDTA enzymes acid

Table 11.4 Types of beta-lactamases.

Group of Preferred Inhibited by: Representative enzyme1 súfrate Clavulanic EDTA enzymes acid

1.

Cephalosporin

- AmpC from Gram negatives

2a

Penicillins

+

- Penicillinases from Gram positives

2b

Penicillins, cephalosporins

+

- TEM-12, TEM-2, SHV-1 from Gram negatives

2be

Penicillins, cephalosporins monobactams

+

- TEM-3 to TEM-26

2br

Penicillins

+/-

- TEM-30 to TEM-36

2c

Penicillins, carbenicillin

+

- PSE-1, PSE-3, PSE-4

2d

Penicillins, carbenicillin

+/-

- OXA-1 to OXA-11

2e

Cephalosporin

+

- Inducible

cephalosporinases from Proteus vulgaris cephalosporinases from Proteus vulgaris

2f Penicillins, cephalosporins, carbapenems

3 Most ß-lactams, including carbapenems

Penicillins

NMC-A from Enterobacter cloacae, Sme-I from Serratia marcescens L1 from Xanthomonas maltophilia, CcrA from Bacteroides fragilis

Penicillinase from

Pseudomonas cepacia

Based on the Bush-Jacoby-Medeiros scheme (1995) 2Plasmid-encoded ß-lactamases (TEM, PSE, OXA, SHV)

Kanamycin (11.27) can be modified in at least six different ways. Thus, the development of aminoglycosides such as amikacin (Section 11.6.1 (11.28)), with considerably greater resistance to many of these enzymes, is a significant improvement to the currently available range of antibiotics.

11.4.1.3 Chloramphenicol-inactivating enzymes

Some bacteria, notably R+ Gram-negative rods and staphylococci containing transducible plasmids (Section 11.4.3) can produce an enzyme, chloramphenicol acetyltransferase, that acetylates the hydroxyl groups in the side-chain of chloramphenicol (11.29) to produce initially 3-acetoxychloramphenicol (11.30) and finally 1,3-diacetoxychloramphenicol (11.31) which is inactive. In S. aureus, the acetyltransferase is an inducible enzyme, whereas in Gram-negative bacteria the enzyme is constitutively synthesized. The design of new chloramphenicol derivatives (in which the terminal hydroxyl is replaced by fluorine) might have been a major advance, since these do not function as substrates for chloramphenicol acetyltransferase. Unfortunately, these derivatives were found to be toxic.

(11.29) chloramphenicol

OH H H O (11.30) 3-acetoxychlorampheiiicol i

CHC1,

0 0

Post a comment