Lyme disease bacteria are usually considered Gram-negative and a Lyme disease Gram stain is used in identification of Borrelia. Gram-negative bacteria are called this simply because they do not retain the crystal violet colored dye in their cell wall during a Gram stain test. Once a counter-stain chemical is used however, the Gram-negative bacteria cell wall holds the pinkish dye and this is a characteristic feature of bacteria with a cell wall constituting a thin layer of peptidoglycan. Borrelia spirochaetes are not straight-forwardly Gram-negative however, as their double-membrane envelopes have a number of significant differences in structure and composition to other Gram-negative bacteria so defined by their double-membrane structure.
Basic Bacterial Biology
To explain this further an understanding of some cell biology and genetics is necessary. Firstly, a diderm is a double-membrane envelope which is found in Gram-negative bacteria and which consists of an outer membrane and an inner cytoplasmic membrane. Lipopolysaccharides are molecules formed using a covalent bond between a lipid and a polysaccharide. These molecules may also be referred to as lipoglycans and they are usually found in the outer membrane of Gram-negative bacteria. Other important definitions include the term ‘periplasmic’. The periplasmic space (periplasm) is the space in Gram-negative bacteria found between the inner and outer membranes and may account for some 40% of the total cell volume in such bacteria. It is much smaller in Gram-positive bacteria and is essentially the space outside the inner membrane in such bacteria.
Borrelia bacteria, whilst classified as gram-negative, have major surface lipoproteins at the host-pathogen interface during infection transmission, and the periplasmic flagella play a peculiar role in defining both cell motility and cell shape. Lyme disease bacteria are also lacking lipopolysaccharides (LPS) in their diderm, and some cell biologists consider these bacteria as neither Gram-positive nor Gram-negative but their own special bacterial cell type. Despite their double-membrane structure, the genetic make-up of Borrelia identifies them, along with other spirochaetes, as a separate eubacterial phylum.
‘Herxing’ and Endotoxins in Lyme Disease
LPS act as endotoxins and are responsible for triggering a strong immune system response in animals infected with such Gram-negative bacteria. The lack of LPS in the outer membrane of Borrelia burgdorferi has implications for the presumed Jarisch-Herxheimer reaction (‘herxing’) attributed to endotoxin-release during antibiotic treatment of Lyme disease but which may instead be a reaction to the die-off induced by the immune system. A Lyme disease Gram-stain is far from straightforward therefore, and an improved understanding of the Borrelia bacterial structure is essential for proper disease management to occur.