Lab Weeds is the term popularized by David M. Ward, a microbiologist at Montana State University (among others), to describe the often unrepresentative members of prokaryotic (bacterial and archaeal) communities that are easily studied because they grow, and grow well, in the laboratory.
Many environmental bacteria remain unculturable in the laboratory because the proper growth conditions are not known. Some may be syntrophic, depending on cofactors, nutrient fluxes, or ambient conditions provided by other members of the community. Anaerobes may be especially likely to require syntrophic partnerships. For microbes from most environmental samples, the ability to grow and thrive in monoculture on some relatively simple growth medium may atypical.
Even for bacteria given every nutritional requirement, the ability to grow rapidly, like a weed, also is far from universal. Genomic analysis of surface ocean picoplankton suggests the existence of communities of very slow growing organisms that lack the regulatory mechanisms to respond with rapid growth under any conditions (PMID: 21048761). In contrast to the typical lab weed, an obligate slow grower would show reduced numbers of transcriptional regulators, among other features of genomic streamlining.