Antibiotic-resistant strains of pathogenic bacteria are increasingly prevalent in hospitals and the community. New antibiotics are needed to combat these bacterial pathogens, but progress in developing them has been slow. Historically, most antibiotics have come from a small set of molecular scaffolds whose functional lifetimes have been extended by generations of synthetic tailoring. The emergence of multidrug resistance among the latest generation of pathogens suggests that the discovery of new scaffolds should be a priority.

Intrinsic antibiotic resistance has been a fact of bacterium life since long before humans discovered the use of antibiotic drugs. However, the introduction of pharmaceutical antibiotics in the 1940s and explosion in use ever since dramatically accelerated the spread of antibiotic-resistance genes.

Since in-feed antibiotics (IFAs) are being taken out of broiler diets around the world, beginning in Sweden in the year 1986, the search for alternatives to replace IFAs has gained increasing interest in animal nutrition. Gut microflora appears to be the target for IFAs and alternatives to exert health benefits and some growth-promoting effects. In this review the effect of six kinds of alternatives to IFAs on gut microflora is discussed and their working mechanisms and growth-promoting effects are reviewed.

The accelerated growth of finfish aquaculture has resulted in a series of developments detrimental to the environment and human health. The latter is illustrated by the widespread and unrestricted use of prophylactic antibiotics in this industry, especially in developing countries, to forestall bacterial infections resulting from sanitary shortcomings in fish rearing.

Antimicrobial compounds are commonly included in poultry diets for promoting of growth and control of diseases. The European Union banned feed grade antibiotic growth promoters, due to, not only cross-resistance, but also to the risk of possible drugs multiple resistances in human pathogenic bacteria. Only two such drug-related compounds are planned to remain in use (Gill, 1999; Bach Knudsen, 2001; Schwarz et al., 2001; Shane, 2001). Feeds containing no chemical additives are increasingly used in poultry nutrition.

An in vivo experiment was conducted to study the potential prebiotic effects of mushroom and herb polysaccharide extracts, Lentinus edodes extract (LenE), Tremella fuciformis extract, and Astragalus membranaceus Radix extract, on chicken growth and the cecal microbial ecosystem, as compared with the antibiotic Apramycin (APR). This investigation was carried out in terms of a dose-response study. The chickens were naturally infected with avian Mycoplasma gallisepticum prior to the experiment.

Surveillance of bacterial susceptibility to five antimicrobial agents was performed during a 1-year period in and around four freshwater fish farms situated along a stream in western Denmark. Besides assessing the levels of antibiotic resistance among the culturable fraction of microorganisms in fish, water, and sediment samples, two major fish pathogens (88 Flavobacterium psychrophilum isolates and 134 Yersinia ruckeri isolates) and 313 motile Aeromonas isolates, representing a group of ubiquitous aquatic bacteria, were isolated from the same samples.

We determined the association between the use of the glycopeptide antibiotic avoparcin as a growth promoter and the occurrence of Enterococcus faecium (VREF) with high-level resistance to vancomycin (MIC ≥ 64 μg ml−1) on poultry and pig farms. The investigations were conducted as retrospective cohort studies, where groups of farms exposed or not exposed to avoparcin between September 1994 and April 1995 were compared.