Also additives must be investigated by the classical procedure of approval (phase 1 to 3) and it will take some time until a wide variety of these substances will be on the market. Additionally, in systemic infection synergistic effects can only be suggested if the additive has a similar pharmacokinetik behaviour an reach the side of infection together with the antibiotic.
I am approaching this issue from a very different perspective, that when you discover a new antibiotic, do not feed it to the bacteria in a refined forms, so that it makes it more difficult to neutralize the antibiotic effects. For example, I am using crude plant extracts with very good results, that you can see at http://www.ecoseeds.com/mrsa.html and my thinking is, do not purify, and add more than one plant extract, at least three different compounds, like adding a combination lock on the antibiotic.
If we keep sending in one purified antibiotic at a time, and especially when we feed those to domesticated animals that are not sick, just to gain weight, then we will very soon loose effectiveness on most of the antibiotics within another decade or two. We need to stop feeding antibiotics to animals for weight gain RIGHT NOW, and look into substituting PROBIOTICS instead.
Compounds containing synergistic activity with conventional antimicrobial agents (antibiotics, antifungal, antiviral etc.,) against pathogenic microbes, particularly 'drug resistant' microbes are welcome addition. This will be even better if they hold true in the real world (biologically relevant conditions) and less or nontoxic to mammalian cells. As we all know, combination therapy is more effective than mono therapy and limits development of drug resistance.
Drug synergism is promising way to treat drug-resistant bacteria. Good examples include beta-lactamase inhibitors combined with beta-lactam antibiotics; efflux pump inhibitors combined with antibiotics, as well as some combination of antibiotics. This approach can restore the potency of old antibiotics and result in lower possibility of resistance development.
Yes, this is currently done with clavulanic acid, which can prolong the lifetime of beta-lactams and overcome resistance. Another example of synergistic activity currently used are anti-folates trimethoprim and sulfa drugs.
Or perhaps this recent publication highlights the direction you are thinking - such as treating with extracts or even cultures:
http://www.ncbi.nlm.nih.gov/pubmed/25215495
"A systematic analysis of biosynthetic gene clusters in the human microbiome reveals a common family of antibiotics."
Almeida J, Tomé J, Neves G, Tomé A, Cavaleiro J, Cunha A, Costa L, Faustino M, Almeida A (2014). Photodynamic inactivation of multidrug-resistant bacteria in hospital wastewaters: influence of residual antibiotics. Photochemical & photobiological Sciences, 13: 626-633.