Bacteria are now generally believed to adopt two main lifestyles: planktonic individuals, or surface-attached biofilms. However, in recent years medical microbiologists started to stress that suspended bacterial aggregates are a major form of bacterial communities in chronic infection sites. Despite sharing many similarities with surface-attached biofilms and are thus generally defined as biofilm-like aggregates, these non-attached clumps of cells in vivo show much smaller sizes and different formation mechanisms. Furthermore, ex vivo clinical isolates were frequently reported to be less attached to abiotic surfaces when compared to standard type strains. While this third lifestyle is starting to draw heavy attention in clinical studies, it has a long history in natural and environmental sciences. For example, marine gel particles formed by bacteria attachment to phytoplankton exopolymers have been well documented in oceans; large river and lake snows loaded with bacterial aggregates are frequently found in freshwater systems; multispecies bacterial “flocs” have long been used in wastewater treatment. This review focuses on non-attached aggregates found in a variety of natural and clinical settings, as well as some recent technical developments facilitating aggregate research. The aim is to summarise the characteristics of different types of bacterial aggregates, bridging the knowledge gap, provoking new perspectives for researchers from different fields, and highlighting the importance of more research input in this third lifestyle of bacteria closely relevant to our daily life.

现在人们普遍认为细菌有两种主要生活方式：浮游个体或表面附着的生物膜。然而，近年来，医学微生物学家开始强调悬浮细菌聚集体是慢性感染部位细菌群落的主要形式。尽管与表面附着的生物膜有许多相似之处，因此通常被定义为生物膜样聚集体，但这些非附着的细胞团块体内显示出更小的尺寸和不同的形成机制。此外，离体据报道，与标准型菌株相比，临床分离株对非生物表面的附着较少。虽然这第三种生活方式开始引起临床研究的高度关注，但它在自然和环境科学领域有着悠久的历史。例如，由细菌附着在浮游植物外聚合物上形成的海洋凝胶颗粒在海洋中已有充分记录；淡水系统中经常发现含有细菌聚集体的大河和湖泊积雪；多种细菌“絮凝物”长期以来一直用于废水处理。本综述重点关注在各种自然和临床环境中发现的非附着聚集体，以及促进聚集体研究的一些最新技术发展。目的是总结不同类型细菌聚集体的特征，弥合知识差距，为不同领域的研究人员激发新的视角，并强调对与我们日常生活密切相关的细菌第三种生活方式进行更多研究投入的重要性。