Clyde Thornsberry, PhD

20 June, 2018

Resistance is emerging as a major problem for infectious disease specialists. In the last decade, the increase has quickened dramatically, and we are now seeing an overall incidence of 17%. Dr Thornsberry reported results from an ongoing international resistance surveillance study focusing on respiratory pathogens.

US data on Haemophilus influenzae shows that in 1997 to 1998 only 67% of strains were susceptible to ampicillin, mostly due to production of beta-lactamase. More strains are becoming resistant to trimethoprim sulfamethoxazole and clarithromycin, but all strains were susceptible to levofloxacin.

MIC data demonstrates that there are Moraxella catarrhalis strains resistant to penicillin again, all susceptible to levofloxacin.

Using NCCLS breakpoints, US data confirmed only 65% of Streptococcus pneumoniae were susceptible to penicillin, 22% had intermediate resistance and 13% demonstrated high-level resistance. Susceptibility rates for amoxicillin clavulanic acid and cefuroxime axetil were 83% and 73% respectively, while even ceftriaxone had only an 88% susceptibility rate.

There was also considerable resistance to macrolides, and only 68% of S. pneumoniae are susceptible to trimethoprim sulfamethoxazole. Only very rare strains were resistant to levofloxacin.

It is important to assess the activity of agents against S. pneumoniae in relationship to penicillin-susceptibility status. Amoxicillin clavulanic acid and cefuroxime axetil had very little activity against penicillin-resistant strains. Only 22% were susceptible to ceftriaxone. Levofloxacin maintains excellent efficacy against both penicillin-resistant and -intermediate strains.

International trends showed that essentially all M. catarrhalis strains produce beta-lactamase, and are resistant to amoxicillin and ampicillin. However, all strains of M. catarrhalis were susceptible to levofloxacin.

The susceptibility of H. influenzae to ampicillin is also reduced, from a low of 65% in the United States to 94% in China and Germany. While susceptibility to clarithromycin tends to be in the low 90s, the resistance to trimethoprim sulfamethoxazole has increased with susceptibility rates of approximately 47 to 48% in Brazil, Spain and China. Susceptibility rates for trimethoprim sulfamethoxazole in the United States, Germany and France ranged from 82 to 84%. Importantly, there were no strains of H. influenzae resistant to levofloxacin.

There are enormous differences worldwide for susceptibility of S. pneumoniae. In Germany it is 92%, but in France and Spain only a third of strains are susceptible to penicillin. This wide range also applies to amoxicillin clavulanic acid, ceftriaxone and cefuroxime axetil.

In Japan, only 32% were susceptible to azithromycin, but in Germany 90% were. Levofloxacin is extremely active and even in countries like Japan where it has been used for a long time, resistance is never more than 1%, confirming its excellent activity.